Oh hey, this is a good thing to really figure out, so I went ahead and looked into most of the best places you can respawn each Ixupi. Sorry if some of these notes are fairly rough but I hope they give the general answers well enough

ASH -- I went ahead and checked these even though it's not necessary in our current route Office - 2 extra steps into workroom Burial - Step through blue door & turn around

CLOTH Egypt - After opening the crypt, if it's not there, back up just until you can see the Sphynx, and then just keep turning around until you hear the music on the screen with the Anubis statues. It's the easiest place to respawn, but unfortunately it's just for Cloth. Burial - One step after the first turn up stairs (Likely never worth doing) Closet - Will always be there

METAL Movie - Go down the stairs all the way to the secret passage door, then turn around. Animal - Go up to where you're facing the green door straight on & the ladder is still visible (there shouldn't be any animal statues visible before turning around) Bedroom - Go through the bedroom door & turn around.

CRYSTAL Stairs - Look at door to Prehistoric room (don't have to go in) Water - 3 steps into underworld stair tunnel (likely never worth going for)

WATER Fountain - Go through the library door & turn around Closet - The Silver Hallway (Probably can't trigger before Cloth though!)

TAR Animal - Shouldn't ever be worth looking into Under - Will always be there

WOOD Work - Nothing good I could find, not worth going for Hall - Just go to the corner past Van Gogh portrait & turn toward the stairs while turning back. (This can be done quickly just going between two screens, because if you turn past the ixupi location, the music will skip if it's there. Sorry if that's explained poorly) Redroom - Go up to the Blue door (don't have to go through, but you have to go into it's "tunnel") Upper - The Silver Hallway

WAX Candles - Face wall away from Elevator Voodoo - Step into the red room tunnel & turn around Snake - The Silver Hallway

SAND Plant - Not worth Water - 3 Steps into underworld stair tunnel

Didn't bother looking into the Electric respawn points too much

Funny so much discussion is happening for this now that it's over a year and a half after the question was asked. Anyway, I mostly agree with you guys on this, that a good requirement for 100% should involve doing all optional puzzles, the full museum scavenger hunt, and finding all flashback items. I'm pretty sure you only do 2 of each Elevator puzzle variant before it moves on to the next, though I would require doing the 6x6 once because the "difficulty" change seems notable enough, but doing it twice or more doesn't unlock anything further at that point. Additionally, I wouldn't bother with much of what involves the actual score in game, because I'm pretty sure you can just keep doing the elevator puzzles to constantly raise it, leading to no real max score.