"Clones and snapshots".Although I'm still not sure why after destroying the 24.7G boot environment the disk space isn't reflecting the freed up space.
It's simply "something", a snapshot or another clone still holds references to the blocks.
doing beadm destroy and saying "no" when it asks if you want to delete the origin, I think just deletes the clone, but the underlying snapshot still exists, so the space won't get freed until there is nothing left referencing that snapshot.
If you have multiple boot environments that all reference the same snapshot, I think if you answer "y" on delete the origin, it won't delete the snapshot until all the clones are deleted first, so it may tell you "I can't because something else uses it"
The output of the zfs destroy -nv command will give a really good clue.
I'm going to leave VladiBG in charge of the GELI bits (not my area of expertise), so I'm trying to help you understand how the boot environments, the underlying snapshots and clones all fit together.