The first link you posted is a clip from Will Vinton's "Adventures of Mark Twain". He and his lead animator from that film were my animation teachers at school.
The boxing film Don's talking about sounds like Break by Russian animator Garri Bardin:
Is that the one? He's got a lot of great films - the link goes to a playlist featuring many of them. I recommend especially Adagio, Grey Wolf and Little Red Riding Hood, and Puss in Boots, but I'm sure there are others. It's a really dark version of Break, but seems to be the only version I can find on YT.
Thanks for fleshing out your description of the long lost film so well - amazing job of describing it! I almost feel like I've seen it now - in fact, funny your last comment, because literally from your description I again feel like I've seen the film in some lost hazy memory but just can't quite pull it out into the light of day. I hope somebody who has seen it (for real) sees this thread and ends your misery (which has now become shared among a bunch of us!)
Strider- maybe you could ask Kelly! He's an expert on obscure stop motion films, right?
I haven't talked to Kelly for quite a while, wouldn't feel right suddenly contacting him with a question like that. I did ask Sven, who runs an animation festival and has a nearly encyclopedic knowledge of animators and their films, and he was as stumped as we are.
It's not the Quays or Svankmajer, though searching for them might turn up films that look or feel like their work. Hm… who was that one guy, English I think, who did creepy films in that kind of vein? Wish I could remember. He did one with siamese twins who get separated in a hospital room and there's lots of death and murder and blood… what was that guy's name? It could be one of his.
He also did one where a guy keeps looking through a peephole in his wall into a woman's room next door, and she puts on shows for him. I think he's a teacher. Anybody know who I'm talking about?
You might try the Wayback Machine on Archive.org - they preserve old sites that have gone down. You just need to type in the url and if they have any archived versions of the site you'll be able to look at them. I don't think it preserves video, usually anyway, but the written info and pictures should still be there.
In case I didn't make it very clear (which I don't think I did) - I was suggesting you use the Wayback Machine to look at the guy's MySpace page again. Even if the video is no longer posted there, maybe he wrote the title of it or some other info that could serve as a clue to finding it again.