I restored a snapshot and I can drop the tables there, so we'll likely proceed to swap the replicas over tomorrow. I have this corrupted instance i can continue to debug on if necessary. There seem to be some other issues now that we're investigating, like a max(timestamp) query on the old instance is doing a seqscan on all the partitions instead of using an index that's there, the new instance from the snapshot correctly uses the index, so I suspect further corruption.
Thanks for your help!
-Michel