I don't think Puljujärvi for Kostin would make much sense. It would also require also moving one of Blais, Sanford or Fabbri to open a roster spot, and Puljujärvi has shown himself to be problematic when he isn't getting what he wants. So a waiver exempt player for a waiver eligible forward would create a headache. Still, any move for Fleury would only make sense if we decided to move on from Edmundson.
If we're having problems with Kostin, as some are speculating, then I'm not sure he would have too much value.
I wouldn't describe him as a reclamation project. He's not 23 yet and he's had almost 100 NHL games, admittedly taking a step back this season. He's good enough for the roster, it's just whether he's still in 7th Dman territory or not.
While he might not be in their plans due to their wealth of talent on defense, I'm sure other teams will be interested.
I can understand not wanting to move Kostin though.