1) Not as such. I mostly let people pitch things at me. The only time a pitch was directed by me, it was more in terms of where to go with the characters. I only did, for example, a Cloud City story because Phae Weldon pitched one. Had someone thrown me an Atavachron story or a Doomsday Machine story, I would have loved to have seen it. (Having said that, I'd be more wary of a Doomsday Machine story as both Peter David in Vendetta and Dean Smith & Kris Rusch in the comics series Planet Killer already did that.)
2) Mike Collins, and no.
3) Well, the two- and three-parters pretty much were novel length. But no, not really. The novella length worked nicely, and the longer ones just got split into two parts. (Or, in the case of Foundations, three.)
4) Dave pitched Wildfire as a major disaster movie, and he, John Ordover, and I all agreed that, if we were going to do this, we should do it for realsies. So we wanted to kill a major character as well as a lot of secondary characters -- and, more to the point (and this was my insistence), the consequences had to be long-term and real for the crew. The deaths in Wildfire continued to have an impact on the crew all the way to the final story in Remembrance of Things Past.
5) Not really. They were all pretty fabulous, and even if I had trouble with any of them, that ceased when some writer or other did them perfectly.
6) Oh, probably Ilsa Bick in the bar at Shore Leave convincing me that making Lense pregnant would be awesome because she'd be such a terrible mother...........