I'm trying to remember something from 5 months ago, so forgive me for being light on details 😄
I believe the mapping difficulties I was having were related to USB identification order. I have a 4-player cab setup with Sanwa sticks, but two different boards for input (P2 and P3 share an input, P1 and P4 share an input). However, P1 and P4 don't have the full 8-button treatment. They only have 4 buttons (in addition to coin/start). So on board1, 7+8 was mapped differently than for board2. I think my workaround was to switch from 7+8 to 0+4 (which would guarantee that all players would have those inputs available to them). If memory serves me, you also need to set this up as an association with the emulator in LB but I could be wrong on that one.
To make matters worse, the USB read order was getting screwed up on system boot, so sometimes board1 would read first (correct) and sometimes board2 would read first (incorrect) and make controlling BB impossible. Adding in the two Sinden guns just make it SOOOO fun 🙄
Luckily, I've since sorted all that out! Best of luck!