My opinion about Zack coming back. My guess is it is just for a visit. Weather you people like or hate him(i dislike him myself) you have to admit it is a good twist to get viewers just to see what will happen.
As for keith wanting Scott i do agree with him, if it ever comes down to people voting, Scott is the most likable. I think they are just though trying to show tension between the new big 4 though because otherwise the show would be obvious to how they would pick people off.