This is very interesting, as a scenario, actually. What will actually happen, I think, if this depowered Justice League were trapped in TWD universe (and couldn't get out) survival would not be the problem for long. They would be trying to save the human population. It becomes much bigger than that. They would be worried, not about themselves... zombies aren't a threat to them for long, but they would try to create a safe environment for as many people as they could. Batman and Diana would probably be training an army of Robins and Wonder Girls in short order, too.