Re: Voldemort teleoperating Quirrell... if there's a quick summary somewhere of why this is a plausible explanation for Q's occasional zombie mode, I'd love a pointer.

Re: remote-snakeform... if that's what's going on, it would be better writing to introduce the possibility of remote animagusing somewhere along the line. Lacking any such introduction (or have I just missed it?) it seems a far simpler explanation that Harry's current companion and Harry's DODA instructor share a body, and that body is a snake animagus. (cf hooves, horses, zebras)