So first, I m not the dev of this application, so it 's just my opinion.
For me it's simple, this application is not optimised for computer but for cabinet or arcade dedicated machine.
You have only few button disponible on your cabinet or your joypad, so the same button have many action depending on actual working mode.
Why it's not standard windows mapping ? For the choice I think it's the basic Mame mapping by defaut, at the origin mGalaxy was optimised for it, now it suport more system, but I m sure 90/100 of the users who use this application have Mame on it. I think too some special cabinet interface (like the I-Pac) have the Mame mapping by defaut
No mouse gestion because few system use it, and the interface application is optimised for joypad, I don't see what you want to do with a mouse on it ?
Why alt+f4 doesn't exit and esc too, because you can use this shortcuts on some system, so it's not realy friendly to close mGalaxy just if you want to close a psx emulator.