Hey, I managed to find the fix for #1! I setup logging as written
here, and at least in my instance the lockup was happening when trying to read the MAME-2003Plus cheats.dat file.
Mine was located under /retroarch/system/mame-2003plus/cheats.dat, which I just completely deleted and now it loads up fine! So check out if you have any cheat.dat files for any of your MAME cores located anywhere (I didn't realise they could also be located under the /system folder and delete or rename them.
No idea if it still happens with cheats.dat files downloaded directly from the Retroarch online repo or just whatever old dat I had on my Switch, but everything is definitely running better now