Sorry, I took the rest of Christmas day off!
Well, your instructions are clear for getting this working in non-Qubes-Whonix, so I can write something up for that and check with you before editing the wiki stub.
There is something obviously wrong in the Qubes-Whonix environment though, so I think we can just note that this is ‘blocked’ pending resolution of Qubes Issue #2540.
This is a real bummer, because with over 600 issues, this may not be solved anytime soon. Or, bubblewrap may have some incompatibility with the Qubes environment.
Thus, Qubes-Whonix users may need to resort to the inferior Firejail as an interim sandboxing work-around, until instructions in that other thread is sorted out and tested.