Reinhart won't be moved IMO. Elite hockey IQ + very strong overall skillset, he could become a very very solid two-way pivot behind McDavid, giving them a 1-2 punch up the middle that ranks top 5, or even top 2-3 in the entire league.
Girgensens has looked pretty solid, but his ceiling is considered lower than Reinharts. I think we could maybe get Girgensens+ from Buffalo, but not Reinhart.
Problem is Buffalo is probably willing to move a lot of solid B-pieces (Grigorenko, their late 1st from this draft, 2nd rounders, prospects like Compher, etc), but they don't really have any solid central pieces to a Kessel trade.
They're not about to move their 2015 1st, Reinhart, or any of their young blueliners IMO. Without a stud on the blueline, they kind of need to rely on the depth of youth, and hope that 1-2 of them take steps forward to become a solid top pairing.