1. Bane and Talia's plan is to watch Gotham tear itself apart. Bane says he wants to give the people hope (letting them live as the bomb's reactor slowly decays) and Talia wants to finish her father's plans (which if you recall from BB was to turn Gotham on itself using chemically induced fear). They could've blown the bomb when they first saw Batman return to Gotham but I'm guessing Bane wanted to defeat Batman and crush any hope Gotham still had for a return to normalcy. As for Talia not blowing the bomb, I think it's likely she was either unaware of Batman's return until it was too late and Batman had taken out the detonator device, making it impossible for her to detonate it.
2. This is a fundamental part of the movie. Yes, the street criminals were gone, in exchange for that safety the corruption that once plagued the streets now plagued the city's laws (The Dent Act locked people away with no parole and stripped criminals of their rights). The truth is that Batman did not save Gotham in BB, he only pushed them toward a new type of corruption, one more deeply entrenched in society (which went on to produce the Joker in TDK). The point of the attack is to take down the corrupt city which has sacrificed its core values of freedom in exchange for security from crime. The act is driven by Talia motivation to avenge and complete her father's mission, but the reason for it is the same Ra's in BB -- to destroy the corruption that was eating away at Gotham.
3. You need to make a better definition of the word criminal and what it means to be a member of the LoS. The LoS IS in fact a criminal organization, they want to commit mass murder. Also, are you referring to the criminals being released from Gotham's prison, because if so, those criminals were definitely not a part of the LoS, they were part of a plot to have Gothamites turn on themselves, forcing them to live in fear of both the bomb and the people they share their city with.
4. He finds the cave because Batman cannot die, Batman is more than a man, Batman is legend. Bruce Wayne can fake his death, but Batman cannot. JGL finds the cave because Bruce knows that thinking you've won and defeated corruption is the only window corruption needs to slip back into society. He knows that corruption isn't gone, it will simply change, and that when it becomes too much for the city to handle on it's own it will need a new Dark Knight to rise up and take Batman's place. Whether he uses the alias Robin, Batman, Nightwing or something else entirely doesn't matter (that's for fan-fics to decide on their own).
Also, he finds the cave using coordinates and gear that Bruce left him in large black bag. I love how JGL following the coordinates and repelling into the cave mirrors the scene where Bruce climbs down the well and finds the cave the first time himself.
5. They could have made their own bomb. But what is central to Bane/Talia's plane is that Gotham destroys itself. Not only that, it was a fusion bomb which is newer, much larger and destructive technology. Also, they needed a bomb that could only be disarmed by one person (Dr. Pavel).
Great timing.
