[suicideneil]

Your batts will be fine so long as they arent actually touching (air is a poor conductor of heat).
Using an external BEC will take some load off of the esc with regards to powering the Rx and servos, but without knowing exactly what causes some MMM becs to die exactly, at best it wont make a blind bit of difference really; if the esc is going to die, theres nowt you can do to stop it, apart from from very tricky internal pcb modifications...