Separate names with a comma.
Please create a new backup and to a test restore with this version, is this working?
I am not aware of such a bug. We had one in the beta of 5.0, but this one was fixed fast.
I assume you missed updating your hosts.
As long as you ZFS as a directory storage, then you can store any file, also qcow2 files. But this is NOT recommended, see above.