I had pretty much the same premise as what you have described though, with the additional add on that Ignis is actually a Tenebraen, but Lucian by Royally decreed citizenship. My thoughts are that House Scientia originally served the Fleuret family for generations, Ignis grew up along side Lunafreya and Ravus. Originally Ignis was to be a steward of the Oracle - Lunafreya’s mother Sylva at the time, learning of the prophecy, history, and duty can really mature a kid.
Ravus and Luna would have treated Ignis as a younger brother much to the ire of Ignis (He would have been a precocious child). It was in Tenebrae that Ignis developed his accent like his parents (the official reasoning that Ignis is always in Court in Lucis is BS to me, if that’s so Clarus should have a pronounced accent and Gladio should have one somewhat as he also looks up to Regis as seen in Brotherhood). Ravus and Ignis would have sparred together - which links to why Ignis and Ravus are able to do chain attacks in Episode Ignis.
Skipping ahead to when Tenebrae gets attacked by General Glauca, it was then that Ignis lost his parents in the attack and Clarus rescued Ignis. I think Ignis’s eyesight “issue” is psychosomatic, it was this attack that triggered his need for clarity in his personality. Injured and still healing, Regis allowed Ignis to remain in Lucis for his protection due to the fact that he worked closely with the Oracle and that Ignis’s only living relative was his uncle (Regis’s steward). It was then that soon after Regis voluntold Ignis to look after Noctis.
I think the main thing for me about Luna, is that so much as riding on the fact that you (as in Noctis) had an exchange diary and yet you can’t read any pages or see any of the contents to get a sense of your relationship with her. Had they shown us the exchanges, Luna would have come off as a better developed character than what we got in the game. What they gave us in Kingsglaive was a treat for Luna’s character, sassy/flirtatious/smart she knows that manipulation is something she has to do to survive her situation of being a prisoner of war of Niflheim. They did somewhat use that in the game where she sort of manipulated Camelia into getting her way to hold the ceremony but they could have handled it better with more fleshing out of Luna’s magical abilities and let us see her kick some Niflheim ass, fight back at the prophecy at least once to show that she tried. The fact that they just make her just go through with the prophecy without even trying to find if there’s a way out is just really frustrating.
For Regis I don’t really find much issue with partly because I played through A King’s Tale and Platinum Demo that gave us a peak at the kind of relationship Regis and Noctis have which was cute. Though again with what I stated above about Luna, I wish they could have shown Regis trying to fight against the prophecy at least once, or trying to see if there’s another way for his son.
Though on the grand scheme FFXV is basically a game with the concept of acceptance of fate at its core. It’s a game that in my opinion shows, given the consequences and the results, these characters have thoroughly decided to accept what may come. It’s a different FF where I believe most FF’s have our characters fighting against fate, whereas FFXV is the complete opposite of it and poses the question of “What if our characters decide to accept their fate.” Yes it’s quite bitter to take straight up and that’s why I think the devs decided to push the brotherhood and buddy aspect of the game, to somewhat soften the ugly yet sort of beautiful in a sense end in the game.
Sorry this got really long >_>;