Re: Why did the League of Shadows want to destroy a peaceful city?
I think it also probably took the League some time to regroup and figure out how to proceed as an organization after Ra's' death. It's not like Talia would have immediately assumed control of the League, seeing as she had ceased her involvement by that point due to her daddy issues. You could make the argument that the League we see in TDKR is a completely different iteration, perhaps comprised of Bane's own mercenaries. It's all a bit ambiguous.
But yes, BlueLightning is right. Regardless of when the LOS became reorganized and had decided on their course of action, because of the crucial role the energy project played in how they wanted to hold the city hostage (it was the most viable way to get a nuke onto the island without any chance of alerting the government), it was something that was going to take a long time to put into effect. The slow knife cuts deepest, as Talia says. I mean, in real life terrorist acts do take years to plan. Come to think of it, the first attack on the Twin Towers in '93 and 9/11 were 8 years apart. So there's a parallel.