Was about to power down the server successfully (interrupting a parity check) and replace the wireguard config folder with the one from backup. Still in the process of getting the tunnel correctly set back up, but marking as solved as I'm now able to access the webgui after doing so.
Edit: of course that was not a simple solution as it isn't allowing me to ping ANY added tunnel now... deleted the 'wireguard' folder inside config on the boot drive, hoping this will allow me to use Wireguard again.
Edit 2: Lo and behold, this was probably all temporary server issues on Mullvad's side. Tried a completely different location which worked right away. Not sure if there was issue any issue on my end to begin with.... as usual, basically my bad and user error