I’ll try to check your logs in a bit, thanks for including those.
So the issue is happening in the NAS to B2 backup, and that issue seems to be a pile of null object errors. My first guess would be some sort of permissions issue - are you running the NAS backup as tray icon, service, or both?
Regarding your plan, I suspect you won’t see much fertilization of the NAS backup, particularly as each machine backup is likely encrypted so the likelihood of encrypted blocks from different matching pad pretty low.
Additionally, you may not see much compression benefits as source files will all be compressed already (unless you have turned that down / off).
That being said, if you don’t care about a second layer of deduplication then your plan should work just fine. Though a restore from B2 becomes a bit of a slog as you have to restore ALL of the B2 backup just to restore a single file to a source desktop.
Well, technically you could figure out all the B2 archive files that contain all the blocks needed to restore the NAS archive files that have all the blocks needed to restore a single desktop file…