I'm gonna have to disagree with you here. Haven't heard Three from the Vault yet, but by 1973-74, it feels like Billy's trying to fill in all of the places that Mickey might have played. I went back and listened the soundtrack from the Grateful Dead movie the other day, and Billy was almost unlistenable in spots.
Glad to be seeing good reviews of Three from the Vault though. It's on my wish list, and the tidbits I've heard here and there, I like quite a bit.
I would disagree with most of what you said of Kreutzmann being unlistenable in parts, but there are times on disk two, during the monumental He's Gone>Jam>Weirdness>The Other One>The Spanish Jam>Mind Left Body Jam>The Other One section that it's obvious that Garcia & Kreutzmann are not on the same page, in fact, not even in the same library. Even give those moments, I still like Kreutzmann solo, seems to give the Dead a little more breathing space. I'll have to listen to The Movie Sound Track again a pay more attention to what the drumming is like.
I always prefer Bill alone with the band. Not that much a fan of the two drum band. I enjoy Mickey alone with the guys too (Mickey and the Hartbeats for example).