"i get the vibe that this is meant to be another Xigbar Isnt Really Luxu Theory" it is! Although most people prefer to say he's actually Brain (?)
The arguments I've seen mostly boil down to 1. KHUX never showing what happened in that fight between Luxu and Darkness, leaving that one unaccounted for, 2. Luxu calling out Darkness for bodyhopping, 3. Darkness being the only one who could reasonably fake being Luxu knowledge wise, 4. Darkness also being smug and confident, 5. Darknesses with personalities like Vanitas existing and 6. Xigbar's bit about Ventus' angry look being better explained by him being a Darkness who shares hive-mind memories of possessing and fighting Ventus in KHUX than by Luxu watching Ventus from afar or their one interaction in BBS.
Other than that it's mostly your standard fare of "but Luxu and Xigbar have differences in personality", which like you said keeps getting treated as an impossible development as if we don't see MUCH more extreme cases of personality shift that no one's ever complained about, like young/old Eraqus, Lauriam/Marluxia and Elrena/Larxene. Saying he's a Darkness also leaves the issue of what happened to "actual" Luxu, of having to explain why he's seemingly more morally complex than e.g. Vanitas who explicitly can't go against his nature as an embodiment of negativity, and the question of how that adds anything more interesting to the narrative than him just being Luxu does.
Anyways, I've seen like four? five? posts about it on multiple platforms so I thought I'd ask, but I hope you're having fun talking about it, too, and that it doesn't feel like just rehashed discourse.!
yeeeeeeep thats what i figured. the Brain thing i have seen and also don't understand.... you already said all the most important points: these Xigbar Isn't Luxu theories raise more questions than they answer, those "answered" questions didn't really need answering in the first place, and the theories aren't more interesting than Xigbar just being Luxu.
frankly i think a lot of this is people speculating wildly and then getting attached to a headcanon/theory, or trying to Guess the Twist Before It Happens for internet points or what have you. it doesn't have anything to do with like...the themes, or the characters, or the things the game says, or even the games themselves, really.
i could come up with specific arguments against everything, back it all up with in-depth analysis of the games, and from the kinds of arguments i see for other poorly-founded theories someone can still go "but if you ignore how stories work and imagine scenes that dont exist and misinterpret tons of cherry-picked dialogue, then my theory is the ONLY possible explanation!!" at which point it's just like. yeah buddy. sure. if you do all those things then it really is the only possible explanation. have fun with your internet points