@SuperFromND Alright, I'll try it. Thanks again!

Ok update as I seem to have managed to get this working. The key for me was to follow all the instructions from @SuperFromND at the top of this post.

Once complete, when i tried to load the game i got "No config found for system mugen" and I was sent back to emulation station.

I eventually ended up editing my etc/emulationstation/es_systems.cfg file as per the below

Replace

<command>/opt/retropie/supplementary/runcommand/runcommand.sh 0 SYS mugen %ROM%</command>

with

<command>/opt/retropie/supplementary/runcommand/runcommand.sh 0 PORT mugen %ROM%</command>

Restart emulation station and to my amazement - it worked!

retropie supports neither mugen nor the switch, so...

Wine only emulates the operating system, the PC Diablo binaries are still x86 so will never run on a Pi. If you want to play Diablo, there is a PSX port.

Yes, N64 emulation is crap for many games, the Pi is weak and emulation is not that good anyway. A few games work well such as F-Zero X.

There are apparently some improvements to N64 emulators coming so maybe the situation will get better in the next few months or year.