AUs where someone other than Siffrin gets stuck in the loops are so fascinating to me because like. we have no idea what a non-looping Siffrin would be like past the Death Corridor. Like, we can assume his mental health would comparatively be doing a lot better, but it's not like he'd just... stay the same the whole time, either. Especially since, remember, it's his job to look out for traps and keys and the group cannot progress if he misses something. And he always misses something the first time around.
Which, in this situation, means that Siffrin will miss the same things, every single time.
So whoever's looping would probably, at some point, start helping to look for stuff, and maybe even get impatient with having Siffrin lead them around all the time, when he doesn't know where to go nearly as well as the person looping. And oh boy, wouldn't that have some fun effects on their perception of themself and their role in the group.
(Granted, they have to be asked by Odile to lead the group through the House, they even seem a little exasperated by it, but unless whoever's looping counteracts that, they're gonna be leading at the start no matter what. Trying to take that away from him after the fact would be... A Situation.)
And of course, de-centering them from the protagonist position puts them in an inherently different light. It's interesting; Siffrin feels like he wasn't meant to be the protagonist, because that's clearly Mirabelle, but at the same time, I feel like anyone else looping would be like. fighting him the whole way for that position. And it all comes down to the fact that Siffrin leads the party. Siffrin will be asked by the King, every time, if they remember.
Siffrin will, probably, still throw himself in the line of fire when the party loses to a Sadness, putting him in the role of "the one who the time looper has to watch die the most."
Plus, you can do so much with how the time loops interact with the main character's personality, the same way they did with Siffrin. Like, assuming it works under the same rules as "time loops back whenever you feel like you can't continue," what would make the others feel that way? Probably not the exact same things as Siffrin, because Siffrin catastrophizes to a degree that most of the others probably do not, so you get to come up with all sorts of brand new, harrowing scenarios to put them in.
Fun stuff! I love seeing it! Please don't worry about making a story like this just because you might think it's redundant, we can always use more!