I guess they are leftover orphaned blocks that still reside in some nodes' mempools.
No, they aren't. Full nodes store the orphaned blocks on disk along with the rest of the blockchain. They do not reside in the mempool.
They will be discarded eventually.
No, they won't.
Full nodes keep a copy of the orphaned blocks that they are aware of.
You can reinstall your node and let it resynchronize with the network if you want to clean out the orphan blocks that it is storing, but as has been pointed out, there really aren't very many of them, and they aren't taking up much space. You'll go through all that effort and free up maybe 20 MB of space? Why bother?