Re: on the Luna and Noctis having a moment before the Leviathan rite.
I don't think them having one moment secretly meeting prior to the rite in chapter 9 changes the whole motif of them only being able to be together in death, as a moment like this would be fleeting, and only further influences that something like this is their one chance to be truly together for as little as they can because of all the external circumstances weighing on both of them at that moment.
Like what was mentioned earlier, there is actually a period of time in Chapter 9 where Noctis knows exactly where Luna is after his meeting with Camelia, and with a day passing until the rite, so say for example he did sneak out to meet up secretely with Luna at night and escape the manor for a bit to go somewhere together, like to the edge of Altissia or something, maybe had a moment like this in the images, but also knew they had to go back before Dawn so as to not raise suspicion of the Altissia government that was supposed to be holding her. Then the next morning when Noctis see's her from the crowd up on the podium they give a knowing nod to each other, the context of that scene still remains the same even if the had a little moment like this, or whether they hadn't, because it's Noctis looking proudly up at Luna, but if they had that little moment beforehand it could give it a lot more.
And on that note, the above picture was drawn by Yuki Matsuzawa based on a concept that Yoko Shimomura told him, and it was used as the cover for the Piano collections.
As well as this companion art
The 1st picture actually has a bit more to it, with a piano by the lake.
And this new picture of Noctis and Luna they showed when announcing the 2019 DLC had Noctis and Luna as kids playing the piano, with Shimomura confirmed to be making a new theme song for it.
So I wonder maybe they could have something like Noctis and crew heard about some sort of tale about a ghost piano or something near a lake somewhere in Accordo, and when sneaking out that's where Noctis and Luna go, and could tie in to any potential flashbacks to their childhood spend together and is symbolized with the piano motif and Shimomura's new theme song.
Either way I'm expecting some new flashbacks scenes with Noctis and Luna regardless, otherwise why else even show that image of Noct and Luna playing piano when announcing the 2019 DLC.
The main issue with Luna isn't her lack of screentime either, also earlier mentioned, it's that not enough of her interaction with Noctis in particular is shown for us to get a good enough idea of their relationship. And I disagree that she is not a good character, her character is fine, the issue isn't her character, it's that she isn't seen interacting with other main story characters enough outside of Gentiana and Ravus.
As far as non party member but story related "princess" type characters go in JRPGs or Japanese games, Luna actually has pretty much average amount of screentime, especially in comparison to Zelda or even say Kairi in KH1. Like Kairi has less than 30 minutes of screentime in KH1's story, and Luna has just over that amount in XV, not counting anything from Kingsglaive or her actual 3 minutes in Brotherhood.
Also these versions of Sorrow Without Solace and Disquiet should be used in the game somewhere.