nice name,did your mom make it up,or you earn it?

Re: 1st question. I'm pretty sure the mouse-button thing is hard-coded, I'm afraid. The MAME authors really didn't seem to design the mouse controls to be very configurable. In fact, mouse control with the arcade games doesn't seem to be a very high priority to the coders of MAME; the control of your marble in Marble Madness is too slow to make the game possible to win, and the light-gun control for "Turkey Shoot" was never implemented until years after the game started being fully emulated.

Re: 2nd question. More than likely, you saw a screenshot of a different version of Metal Slug. If not, maybe only certain characters spit out red instead of white?

- Tricob.