It turns out I answered my own problem in that thread regarding sudoedit in dom0: Monero Wallet/Daemon Isolation with Qubes + Whonix - #79 by tanky0u
EDIT: No chance! What the hell! Even with
SUDOEDITOR=/usr/bin/vim sudoedit /etc/qubes/policy.d/30-user.policy
I am still getting the same nonsensical error message I posted above.