Deswizzling is one of those odd things. Either you let it finish or it never finishes. Since you're wanting to use FabricPool, you'll need to let it finish.
AFS first can help, just make sure you're on a recent release as some of the early ONTAP 9 had a data corruption bug. Basically yes you have to stop ALL snapshots. It sucks but you do.
We have had BURTs in the past about vol move, but honestly if you want this to be prioritized the best bet is to talk to your account team. They have more power to help here in the long run.
I agree, it's a sucky situation and it would help if it used XDP instead.