I know I'm somewhat late to the part, but anyway:

Considering infighting, unless an Uchiha is fighting a blind opponent, he can simply take an eye from the fallen one to replace his own Izanagi blinded eye. So, one can indeed use it infinitely. The added bonus is eternal Mangekyo. What comes to them all having Mangekyo, I suppose during the more turbulent times, they all kept losing friends nonstop (or rather, had their share of grief and stress, because losing a friend doesn't actually mean anything inherently) and they all indeed needed the Mangekyo to fight against other monster ninja of the old times. So, Mangekyo wasn't some big secret locked into a basement but something every capable Uchiha was taught eventually.

Still, that flashback gave nice background to why the rest of Konoha was always so wary of Uchiha: The lot kept killing each other for petty power and position, which doesn't exactly create trust.