RetroPie Mame-Libretro key mapping and [TAB] config window

[you2industries]

I have recently installed RetroPie on my Raspberry 3, and it has been pretty good with just a few snags.

My next step is to remap the 'insert coin' button, which is currently [SELECT] to a different key, namely [C] on the keyboard.

At first I was unable in insert coins into any ROM, but after switching from Mame4All to Libreto Mame and re-assigning the [SELECT] and [START] keys just using the Emulation Station key assignment interface, I was finally able to insert coins and start most of the ROMS.

Now, I need to remap the 'insert coin' key to it's own key, that is, a separate key from the [SELECT] key. After researching, it looks like I need to access the Mame configuration interface, normally accessed by pressing [TAB] while running any ROM, but I've had no success and most of the suggestions to delete the default.cfg file located in /opt/retropie/configs/mame-mame4all/cfg or ~/RetroPie/roms/mame-libretro/mame2003/cfg among other locations aren't helpful because that file doesn't exist in any of the suggested locations.

So,

• 1. How do I remap the 'insert coin' button to its own key?

and..

• 2. If I need to access the Mame config dialogue, how do I fix the [TAB] key mapping?

Thanks in advance :)