Pulling my hair out - MAME keyboard controller configuration

Got Batocera v41 working pretty great with a standard def CRT and MAME in my arcade cab. I’m using a very old Ultimarc iPAC4 USB controller which doesn’t support xinput or fancy things like that. It just maps to keyboard button presses. The Player 1 and 2 keys map fine except to Player 2 buttons 5 and 6. The IPAC outputs those as I and K, but the MAME default config in the MAME config menu (tab button) shows E and none. I can change them, but the changes don’t persist once I restart the game, even with the mame.ini writeconfig as 1. I tried adding these to the default.cfg file, but this also gets overwritten every time I restart MAME. Any pointers on how to make these key mappings persistent? I’ve been digging through docs and forum posts for hours and I’m at a loss.