Detective Garza is actually Sgt. Charles "Carlito" Grey.Garza is a relatively minor character, but he always seems to be right about his guesses, is able to get to the scene of the climactic assembly scene in record time, and oh yeah, his specialty just HAPPENS to be Explosive Ordnance Disposal... just like the erstwhile sergeant. If you factor in that Clyde had an intelligence background, he seems the perfect kind of person that Snake Doctor would want kept tabs on.
Clyde is an alternate version of the Riddler.
Clyde is an alternate universe version of Light YagamiBrilliant mastermind arranges his own imprisonment as part of an overly circuitous plan for the sake of great JUSTICE!!! Check.
Clyde somehow survived the explosionand moved to Gotham City where he became known as the Joker. Shrapnel from the explosion of his own bomb is where he got the scars. He is continuing with his goal to tear down the system but has also learned his lesson about how pathetic attempts to control things really are...
Nick's gun isn't loaded.He just wanted to appear more threatening.
Clyde dying at the end was part of the plan.All through the movie Clyde tries to hammer one point into Nick's mind, that the system is flawed and you have to take justice into your own hands. No matter what Nick does, he fails the entire movie, until he takes the law into his own hands, and murders Clyde at the end. That's what Clyde wanted the entire time! He succeeded, he took his revenge, he made an example of the justice system, he converted Nick to his own way of thinking, and above all else, he died, because he wanted to. That's the best part of the gambit. Clyde didn't get outwitted by Nick, he forced Nick to kill him.