He did try to unlock the suitcase but realised he couldn't and couldn't do anything within seconds to save him.

Also the bomb disposal cop said: "This'll take out the entire floor and and the one above us."
I expect they didn't modify the bomb, they just took it to Clyde's cell. If the bomb was powerful enough to destroy a couple of floors of the city hall, DA Rice and detective Dunnigan would not have made it out of the tunnels alive. They would and should have died in the so called explosion.
However I don't think the bomb was the right type either. Napalm (a cop said: "that's napalm, basically") is for burning stuff, not for rapidly destroying load carrying structures. A black ops guy like Clyde surely would have made a better and less detectable bomb in the first place. Such a stupid way to end a very good plot.

OH COME ON!!! I haven't seen it yet! Now I know a bomb will get him in the end! What's wrong with you?
I'm just teasing, I've seen it. I just couldn't resist with your tag "Don't read if"
Checkmate!
Deutschland hat die Weltmeisterschaft zum vierten Mal gewonnen!

I love most of this movie, but I agree that the ending sucks.
Most of the movie establishes what a genius Clyde is, but then he makes a giant blunder? Yeah right.
Furthermore, the ending is made even more unrealistic by the scooby-doo happy ending with Jamie Fox sitting at the dinner table with his family. He would end up in prison. Let's think about this..
A fire bomb explodes inside Clyde's cell. There's going to be an investigation. Once the tunnels are discovered, investigators will learn that the secret door was jammed shut from the tunnel-side, meaning Clyde was trapped inside his cell with a bomb. Someone's going to prison.

What I don't understand is why he didn't have censors in the warehouse, and tunnel triggering a camera so he knew people were in there to begin with?

Ditto!
The ending made the movie unbelievable for me!

all they had was an elaborate revenge movie and they blew through that in 30 minutes. they needed something to fill the rest of the space and to make it "not just another revenge movie"
and what they came up with sucked big time

It seems the writer worked months on the 2/3 of the movie, and a day on the ending. It makes logical sense that the bomb would not be mobile. They moved it and did not deactivate it, why? because they couldn't. So is stands to reason that if he made it so that they can't move it, he would've succeeded. What's even worse is that Clyde knew that Nick found the bomb, as he is in the cell and talking through it and such, yet he doesn't think that, "hey, they found the bomb, the bomb is mobile, ergo, bomb might not be there" And even worse is that Nick took the risk of waiting till he activated the bomb.. if the detonation was instant, then he would've died. Stupid Hollywood ending.

I just watched it and thought that too. The whole ending just bothered me.