1) I think that coaching can provide an "aura effect" to a team. An elite coach can elevate all players to enter a new ranking. A bad coach can deflate player value. It's an unfortunate aspect of the NBA where depending on what team you play for impacts your value. Notable examples are Jae Crowder, Avery Bradley, Evan Turner, IT, Marcus Morris, Kyrie Irving.. I mean basically anyone that we've seen play for another team and for the Celtics. Something in their game elevates in green. To answer this question, it's hard to put a number as far as seeding, but just imagine a bucket of players each in 5 groups: significantly under replacement, under replacement, replacement level, above replacement, significantly above replacement. Playing under a Brad Stevens jumps you up 1 level. Multiply that by a 12 man rotation and it could be a huge difference.
2) We have no factual basis to answer this question, but I believe yes. I also don't believe that Brad would join a team like the Knicks, if that makes sense. He seems to be smart enough to understand a bad situation.