Yet another post here without a proper answer, but does a mosfet (switch, presumably?) have reverse polarity protection? An unprotected mod will vent as soon as those batteries go in the wrong way, was that the case or did you have to fire first?
I know these are questions, not answers, but I want to be absolutely sure those batteries didn't vent. Please hold off using them until we know from a mosfet user.
@TrollDragon, any input?