By Groundhog Day I meant Homura should've been as diligent as that hero was by trying to break his endless cycle as she should've been at convincing the others what was going on each time she cycled back in time.
I don't see anywhere in the anime that hints what happens if you renege on the contract or if a Magic Girl simply strikes. Maybe in the manga which I can't read, but I can't find any clue in the anime. Maybe it's an honor thing. And for me, the conversation between Kyubey and Kyoko hints that there might very well be a way to break the Magic Girl into Witch fate, as Kyubey admits he doesn't know how everything operates. I take him for his word that doesn't "lie", just omits some truth. If some kind of Daniel Webster pops up who can break the contract, I can see Homura leading a MG strike/revolt to starve his people of witch food.
Unless the author intends this to be a "dead-end" series of doomed shafted Magic Girls without any hope (the anime folks wouldn't!), I place my bets that there is a way "out".