Popping in as a lurker of Mognet, the "alternate grand finale" i don't believe is supposed to be an alternate universe. The main game itself has openings here and there for the Bro DLC, and Episode Ardyn itself appears to be set before the final battle with him (lining up with old datamined scripts where he drivels on about Ardyn anime, touching the crystal and something "turning black with anger"). The Luna DLC was likely going to be "alternate" in the sense it was to explain how she came to you on the train, how she provided the summons and why she is with Noct in the ending. Noctis's DLC itself probably elaborates further on what happened after he dies. This "alternate grand finale" was never supposed to replace the main ending, and with that in mind, it's highly likely it's meant to wrap up XV's story on a happier note. The normal ending still exists and is still canon, but the original DLC ending may have simply gone further into detail describing the post-death world that Ardyn, Luna and Noct, all parts of this alternate finale, are a part of. IT all seems like they removed things from the story to make the 2016 release, not sure if they'd ever get to touch upon Ardyn or Luna.
The base game was developed to provide players with a complete story for Noctis, the anime being content they couldn't put into the game for various reasons yet it ties in with the Bro DLC; Ardyn Anime was confirmed to be cut content, and we saw snippets of events that go down in the episode in old datamined scripts. It's likely Luna's role if included in the 2016 game would have been ultra minimal, so they cut it out entirely and set their sights on Royal Edition, a DLC not yet announced yet clearly features things that made the director consider the game "complete", including features like the Royal Vessel that were found in the datamine dating to 2016. With the key focus on Noct and Luna in the art, and the fact that Luna had animations and voice work done around EP Prompto's time, and datamined content reveals an equippable Sylleblossom that'd add her to your party (as a changeable character), she was aimed for the Royal Pack, that may have even been Episode Luna once upon a time but they decided to focus primarily on the Lucii and Noct (the reason those three Lucii are on the original cover of the game makes far more sense now). Given they once again felt like they'd be doing Luna a disservice by not telling her entire perspective and simply leaving her as playable with that summoning scene, they set their sights on another round of DLC and figured they could dedicate a whole episode to her.
They originally said Ardyn was plausible as a movie, and in the end tried to give him as DLC anyways; with the anime being made specifically because they weren't gonna be able to do the entire Ardyn story in Ardyn, we can deduce that EP: Luna and Noct were most likely definitely part of the actual story and tie in to the "verses". Regis saw a vision of what happened to Noct and changed it; in EP: Ignis Verse 2, he sees a vision of what happened to Noct and changed it. It's likely EP: Noct would have seen Noctis either changing his future after seeing a vision of how his journey is supposed to end. The book description implies Noct's DLC was supposed to be in the crystal, and in the datamine, info for Terrawars, EP Ardyn and a "Forest of Memories" was included, this before we officially had a Terrawars announcement and before we officially got EP Ardyn announced, and this forest of memories talked about Noctis witnessing memories of Luna and Regis; given they "didn't know what to do" with Luna's cutscenes, this Forest of Memories was likely where we were supposed to see all those cutscenes of Luna, as well as what Regis was doing in the city (perhaps even his POV of Ardyn's invasion). Adding on, Forest of Memories makes me think of Platinum Demo's forest, the world of dreams that belongs to Noctis.
Luna and Noct DLC were never "alternate" and non-canon to the story, they simply changed the tone of the ending by providing us with the fantastical context of the world after death; Luna wakes up after she died with an altered body, and Ardyn is a man who cannot die who can meet Noct in the crystal when he does, and Noct spent time in this cut Forest of Memories to reflect on his journey.
What say you?