What's the lifecycle of an automatically-created snapshot? i.e. is the snapshot garbage? How does it get collected? Things like autosnapshot implement policy to take care of itself but... zfs send one-offs?

zfs send is a strange beast. It's more like differential tar than rsync (i.e. a stream intended for linear backup). zfs is cool because it unifies backup/restore. Have you tried restoring differential tar from unlabeled tapes?