Just playing around with this more, if
route_allowed_ips="0"
on the Wireguard client (Mullvad) is set, I am able to connect to the wgserver. The final issue, which is really just a zero-sum, none of my traffic goes through Mullvad this way.
I have played around with https://docs.openwrt.melmac.net/vpn-policy-routing/#wireguard-tunnel
Routing Wireguard traffic may require setting
net.ipv4.conf.wg0.rp_filter = 2
in/etc/sysctl.conf
. Please refer to issue #41 for more details.
and followed the issue to read
net.ipv4.conf.default.rp_filter=2
net.ipv4.conf.all.rp_filter=2
The only observable change is that if I don't pass traffic to Mullvad, both Wireguard interfaces will handshake with their corresponding peer (Mullvad handshakes, still doesn't traverse the interface).
I've also found another post attempting the same thing, although it appears they have given up on this: