There's zero chance Duren is even close to being available. Stewart's future with Detroit, on the other hand, isn't 100% considering his awkward fit at PF next to Duren, but that's not something they really are under any urgency to figure out this season. There was a rumor the Celtics kicked the tires on Stewart before he was extended, but nothing happened obviously. Wiseman or Bagley are the ones I think they would eventually move, especially Wiseman if they don't extend him. They're both basically blocked by Duren and you could see Wiseman wanting to go somewhere where he has more of a shot at getting minutes. Those two make too much for the Celtics to really consider, however.
Achiuwa is interesting. I don't know much about him besides him being an athlete who can't shoot. The Raptors could go so many different directions that it's hard to gauge what their thinking is, though. Skimming Raptors message boards, it seems that he was slowed by an ankle injury last year and he had trouble playing different roles other than center. They traded for Poetl and then Achiuwa basically had no idea what to do in a F/PF role. His bball IQ and hands are questionable. Basically, he's an undersized C that has had trouble converting to F/PF, which is what he should probably be in the NBA for his physical profile. For the C's, you get him if you really believe he's better than what he's shown, but I wonder if they wouldn't be more interested in a seasoned veteran as opposed to a guy who's still a project.