I sort of understand their reasoning, then, because acting as a reverse proxy for externally-initiated inbound connections is not typically within the scope of VPN services per se, and especially outside the scope of Mullvad's particular mission of offering VPN services for the purposes of protecting privacy and suppressing censorship.

There are other solutions for your use case, with or without a third-party VPN, that wouldn't require port forwarding. There are also other VPN services that do offer this functionality.