I run head scale on a Vps and it has been 100% reliable and provides an exit node for me via that VPS. But you are going to have the exact same issue, I think with your VPN.

You want your default route from the exit node to go over mullivard's interface, but you need wireguard or tailscale to talk to the other clients directly, via their clearnet IP. You can tell the exit nodes tailscale/wireguard client to bind the clearnet interface, but that may or may not help. Mullivard probably takes the default gateway, which will likely cut off the clients. You could set dedicated routes to the IP addresses of the clients, but they can change if those clients are mobile. You will need to maintain a routing table somehow or keep the pre-existing connection active (fragile).

I am sure its well trodden ground, just be aware that head scale probably won't solve your problem automatically. Its a routing and or firewall config issue.