This is the very definition of an unsubstantiated assumption.
The Famitsu quote you provided -- in Google Translate form, mind! -- said nothing about any piece of artwork other than the specific key image of the happy ending picnic that the interviewer asked about.
And yet, you somehow managed to draw from that simple comment the "fact" that every single piece of artwork included in the DotF art book was created for the same reason, under the same circumstances. That's so far from what was said in that quote that I honestly couldn't even believe that was what you were trying to say at first and read your commentary as if it were your own translation until that interpretation became impossible to sustain.
This is why I asked for positive evidence -- I figured that three quarters of your certainty was based in assumptions you'd made rather than in actual statements made by the development team.
What? No? Did you actually read the quote? He said they held competition with the Art Members of the team to create artworks for an ending that they think fans most want to see and would satisfy them, and that one of the drawings that was created was that Ardyn picnic image, which is only being brought up because that was the only one that was shown back when DotF was announced in 2018 which is why the interviewer asked him about that image, so him saying that they held a competition with the Art Members to create ending artworks for DotF and that that Ardyn picnic art was just one of them not only directly implies that there are other art works made for that competition but that Ardyn picnic art is not the only one made for that competition with the Art Members to depict an ending those artists think fans most want to see and would satisfy them.
I have no trouble believing this is the case for this image, and it bears no relevance to the artwork in question.
It's literally directly related to it because that is another artwork that was created by the Art Members and the first line of the description of that vertical image of Ardyn's funeral with Luna in her Kingsglaive outfit is the same line used for the horizontal Noct and Luna at Ardyn's funeral artwork too, that they are both artworks created for DotF as idea's for the ending and that that is Ardyn's funeral. The fact that that artwork of Luna in her kingsglaive dress at Ardyn's funeral says the same thing in the first 2 sentences as the horizontal artwork of Noct and Luna at Ardyn's funeral means that both were created as idea images for the ending of DotF, even if one was created for DotF earlier than the other or whatever, either way all DotF artwork would have only started being made in 2018 when it was first being planned. There is absolutely no way Ferrari would have been involved in that to begin with, and again that artwork looks nothing like his artstyle.
However.
Google Translate is... Google Translate. Its ability to translate grammar is, quite frankly, nonexistent. That's why its quality is a hundred times better when applied to languages like Spanish, which contain similar grammar and syntax to English -- in those languages, it doesn't have to do anything but translate words, since the grammar and syntax are already largely understandable the way they are.
Japanese, in particular, handles subjects in a very different way than English does. That's why a machine translated line like "This is one of the works when an art member was given an idea image about the ending of 'The Dawn of the Future'" is not proof of anything -- its grammatical oddities aren't just a matter of machine translations translating things weirdly (the way a human translator who isn't a native English speaker might) but a suggestion that the translation itself is highly ambiguous.
Does it mean "This is one of the works created when an art member was asked to create an idea image [concept art] about the ending of The Dawn of the Future"? Maybe. But it could also mean "This is one of the works chosen by an art member as an idea image [concept art] to reflect the ending of The Dawn of the Future..." especially since a later line says, "it was planned to include the scene of 'The First King's Funeral' in the ending being further inspired from this art specifically," which strongly suggests that the artwork influenced the ending rather than vice versa.
The best way to resolve this question, of course, is to get a proper human translation of the caption rather than relying on Google Translate for things that depend on grammatical accuracy.
I only used google translate because I couldn't be bothered doing a full translation when I posted it, but that said the google translate is close enough to the intended meaning regardless,
in that the artwork of Ardyn's funeral was definitely created for DotF.
So let's go through it sentence by sentence.
"これは(This is)アートメンバーに(by an Art Member)『末来への夜明け』(Dawn of the Future) のエンディングについて(About the ending)、アイディアイメージ(Idea image)を出してもらったとき(when came up/when put out)の作品(Work of art)のうちのひとつです(One of)。"
The "エンディングについて" means that the idea image artwork that was created was for the ending of Dawn of the Future.
Basically it means "This is one of the works of art from when an Art Member came up with/put out an idea image about the ending of [Dawn of the Future]"
Which means that an Art Member created that artwork for the ending of Dawn of the Future, and given the context from the Famitsu interview of the competition to create endings for DotF by the Art Members that fans would want to see it only further indicates that this is one of the many artworks that were created by the DotF Art Members as their ideas for the ending that fans would like to see.
2nd line is this
アーデンの (Ardyn's) 葬儀 (Funeral) をイメージして(image)描かれました(was drawn)。
"An image of Ardyn's funeral is depicted"
And both these two art works of Ardyn's funeral state this text in their first two sentences, which confirm that both of them were created as idea images for the ending of Dawn of the Future by Art Members.
"これはアートメンバーに『末来への夜明け』のエンディングについて、アイディアイメージを出してもらったときの作品のひとつです。アーデンの葬儀をイメージして描かれました。"
"This is one of the works of art from when an Art Member came up with/put out an idea image about the ending of [Dawn of the Future]. An image of Ardyn's funeral is depicted."
"これはアートメンバーに『末来への夜明け』のエンディングについて、アイディアイメージを出してもらったときの作品のうちのひとつです。アーデンの葬儀をイメージして描かれました。"
"This is one of the works of art from when an Art Member came up with/put out an idea image about the ending of [Dawn of the Future]. An image of Ardyn's funeral is depicted."
These both say the same thing in their first two sentences with only 1 minor difference, the only difference in their first line is that the vertical artwork says "作品のひとつです" which means (one of the works) while the horizontal artwork says "作品
のうちのひとつです" which also means (one of the works), so both mean the same exact thing in that they are "one of the works", so there is no change in meaning in the first sentence.
The 3rd sentence in the horizontal image states this.
もともと(Originally)『エピソード ノクティス』において(In Episode Noctis)、アーデンもまた(Ardyn also)報われる(be rewarded)最期(last moment/death/end)を想定していましたが(was assumed/envisioned)、さらに(Furthermore/moreover)このアートから(because of this art/from this art)具体的に(Specifically)インスパイアされてエンディングに(inspired the ending of)「初代王の葬儀」[Funeral of the First King]のシーンを入れる予定でした。(scene was planned)"
Basically "Originally in [Episode Noctis] Ardyn was also envisioned to be rewarded in the end, moreover this artwork specifically inspired an ending scene that was planned for the [First Kings's Funeral]"
So basically one of the planned endings they were going with for Episode Noctis was Ardyn being being rewarded with a funeral recognizing him as the First King, which is what the horizontal artwork of Noct and Luna at Ardyn's funeral which was made for DotF specifically inspired them to plan a scene like that, but ultimately they did not go with that.
It was already explained earlier that various artworks were created by the Art Members as different ideas for the ending of DotF for what they think would satisfy fans, and these concept artworks of Ardyn's funeral which are not actually part of the final DotF story are just that, other art works created by the Art Members for DotF when they were still planning the ending for it, which is why they all vary in what they depict, with some showing Ardyn's funeral while other showing Noct and Luna's wedding while another showing Noct and Luna with their kids with Ardyn and the bros having a picnic, another showing Noct and Luna on the throne with their kids, etc.
I'm not sure why you seem to think Roberto Ferrari drew that DotF art work when it looks nothing remotely like his artwork at all, including any of his storyboard drafts from 2013 which all still use a flat art style with no blended colours and all still use basic linework, which is not present at all in the Ardyn funeral artwork for DotF, which again was created specifically for DotF which only started being planned in 2018, which was 5 years AFTER Roberto Ferrari had already left the XV team and was no longer contributing any artwork for the game after he left at the end of 2013.
Here is some of those same storyboard panels that Ferrari drew more clearly, they all use flat colours and basic lineart, there is no blended painterly style employed, Ferrari's style is minimal in how he draws in his flat toned more anime-eqsue way, he always uses lineart and basic flat colours, he rarely if ever will blend colours and even if he does it's only for things such as highlights, not base tones or shade, even in more rudimentary form his artstyle still has solid lineart and clear distinction between colours, which does not look at all like any of the DotF artworks which are using heavy amount of blended colours in a detailed painterly style that doesn't evoke any semblence to Roberto Ferrari's artwork.
See here even when doing backgrounds Ferrari still uses solid flat fills with clear distinction between colours, not blends, and every panel of that storyboard he does uses lineart with these solid fills even if roughly drawn.
I don't know why you think this DotF artwork is by Roberto Ferrari, when this DotF artwork is heavily blended with more dynamic range of colours and blended tones with a ton of highlights, shades, tints, textured brushes and clear use of different brush types, and that also uses no actual lineart as it blocks in with the shades themselves. Especially when compared to Kenji Niki's art below who's artstyle matches exactly with this DotF artwork.
Seriously just look at how Gladio and Ignis are drawn and shaded in this artwork by Kenji Niki here, where they were added to the artwork above, and how they are in the same style as the DotF Ardyn's funeral artwork.
Remember what I said above about assumptions? There's no positive evidence here whatsoever, just a bunch of arguments from absence. (Using the lack of detail in a rough draft to suggest that the design for a complicated weapon wasn't finalized is a particularly absurd sort of argument from absence, even.)
My suggestion about the Sword of the Father isn't that it filled the same role back in Nomura's XV, anyway. My suggestion was that, given the other evidence that the artwork was early, there's no reason it couldn't have been created earlier to fill a different role, like Noct's ultimate weapon.
Funny you talk about assumptions yet all you do is that, why are you assuming Sword of the Father existed in Nomura's XV when all the evidence suggests it can only have been created in 2015/ for Tabata's XV? When the changes were made to Regis which is a direct result of Kingsglaive's production which fully ramped up in 2015, and when we first saw Sword of the Father? There is literally nothing to suggest it ever existed in Nomura's XV nor was the concept of the royal arms even a thing there, that was one of the main things that was added in Tabata's XV.
The existence of concept art for Kingsglaive, which required much higher levels of detail for its characters than 2013!XV would have, is by no means evidence that the concepts behind individual designs didn't exist earlier. The two hooded figures in particular look like attempts to change Nomura-esque designs into something that would work in artificial-"live action."
Hey look more assumptions from you with no evidence or anything to back up what you're saying at all, meanwhile what I posted was artwork from the Art Director of Kingslgaive with all of those designs being brand new and some even unfinalized versions of their Kingsglaive design itself, including that Regis design which is a completely different design to the original Regis design in Nomura's XV, so no, the entire existence of the Kingsglaive itself was never a thing in Nomura's XV either and we can see in all pre 2015 CG of the peace treaty scene all of Regis's council only wore suits, there were no tall robed men with giant black square helmets or anything like that, even in any concept art for Nomura's XV we've seen. You are the only one assuming something here, and based on absolutely nothing at that.
Also the level of difference in concept art for Kingsglaive and XV itself are easily comparable, be it for characters or environments.
Concept art made specifically for XV by SE artists.
Concept art made for Kingsglaive by artists and studios that were commissioned to make artwork.
If the concept for "Luna"'s outfit came from the artist, though, that doesn't really match his style on Kingsglaive at all. =P
Luna has been frequently drawn and depicted in different outfits, including those that never made the final cut, so him drawing Luna in a different outfit which neither Stella nor Luna have ever been depicted in before for that concept art for DotF does not mean anything.