So I have some questions about snap shot. It states that you cannot modify the attack dice. Does that mean if Mauler Mithil or zeta leader use it they can roll one extra die? What about auxiliary or mobile arcs? Can it activate with those? And if so, would you be able to add a crit when flying backdraft? Also, can M9G8 allow a re roll?

Adding a dice prior to rolling is not a modification; so if you've an ability that allows that (e.g. Jan Ors pilot); you can do it. You can't modify the dice after you've rolled them for snap shot. You get what you get and you don't get upset (as I tell my children).

However, Snap Shot is a secondary weapon so abilities that only allow an extra dice to be added to a primary weapon attack won't work. Furthermore, as it's a secondary weapon, it is restricted to your primary arc.

Cool, thank you very much

And to answer your last question, if another ship with M9-G8 has you target locked, then yes that ship can "force" you to re-roll one of the snap shot dice. Since the source of the re-roll is not the ship using snap shot the " You can't modify" doesn't apply.