2. I assume that the map has to be tied to the wards of the castle somehow. I figure that between a genius like Moony is supposed to be and two old family purebloods they'd have been able to find ways to tap into the wards.
5. I'd prevent the death of James Potter. Think about it, Voldemort stuns him thinking he can execute him in public for the blow to morality it would be. Voldie dies upstairs anyway and James wakes up to find his wife dead and his son has just been marked for a prophecy. It would change just about everything. I mean really, the brains, power and ruthlessness of the Marauders all united to protect Harry. Heck, they might kill Voldemort off completely before Harry even starts Hogwarts.