you could add a netvm to that common that is common to the 4 other, but block all network traffic for it using qvm-firewall, this will ease networking between qubes but it won’t be able to do anything over the internet
Yes, it’s connecting the qubes without using networking.
So your 5th HVM running a mysql server can be completely network-isolated with netvm set to none.
I think existing answers show how this can be done, connecting the 4
servers to the backend mysql.
On the specific Whonix question the Whonix devs have asked that these
questions should go the Whonix Forum.
The docs have an
extensive section on setting up onion services in Whonix, with detailed
instructions for the Qubes case.
You have not said why you want to sue Ubuntu HVMs. You could do this
using normal template based qubes, or Whonix workstations, but perhaps
you have your (unstated) reasons.
If you need specific help with the implementation, ask away.
I never presume to speak for the Qubes team.
When I comment in the Forum I speak for myself.