has anyone been able to get Mario Kart Arcade GP (or 2) MAME working?
'Mario Kart Arcade GP' runs on the Triforce arcade system which is based on GameCube hardware. Unfortunately the Raspberry Pi isn't nearly powerful enough to emulate a system that demanding.
As for the rest of your issues, You can't just "pop" any old ROM in the arcade folder. You need to get a specific numbered ROM set for the arcade emulator you've chosen. For example, the default arcade emulator in RetroPie is lr-mame2003, which uses the MAME 0.78 ROM set. Anything outside of the intended ROM set for your chosen emulator will usually result in failure. Also, as a rule of thumb, any game north of 1995 has an increasingly higher chance of running poorly on the Pi.
Everything you need to know for arcade emulation in RetroPie is covered here.