Okay, I'm still out of commission, but I thought I could answer at least Jen's questions in the meantime.
Like Toru said, we will have to fight against the antagonists to get out of the prison. They'll be our opposition while most of the other guards are distracted by the surrounding riot.
Odds are, there are other exists than just the front door, but I wouldn't worry about it so much. Whether they're able to bar it off before we can escape depends on how the story shapes up. There's still some creative wriggle room for us, so we don't have to think about this part as much.
The prison, as far as I know, is above ground but made entirely of metal with little in the way of windows and outside light. I imagine only the prison yard is the real way to the outdoors. It could very well be partially underground, but it can't be said for sure.
Like Toru said, we are the Ba Sing Se Underground and we still have a headquarters the enemy has yet to find. That means we're staying put until the city is liberated. We will probably, after escaping the Upper Ring, have to split up to get back to headquarters, but the main objective will be to hide out there. The prisoners that escape with us (which would be Tetsuya, Rin, Shen-Zhu and Joychi) will become Underground members and will split up with us so they can be introduced to the HQ.
Jen, we'll try to get you some water in time for the fight. You're a creative fighter, so use whatever you have at your disposal to slow down the guards and antagonists.
Joychi, Suki's not actually a commander, she's posing as a guard. She's referred to as "Little Commander" affectionately by members of the Underground. There is definite shades of the Boiling Rock in this story, though we are adding our own spins on everything. I really like your movie idea of splitting up the train cars as we try to escape, though.
We could totally trick our pursuers onto one of the cars and then have one of the earthbenders in the group separate it from the others, thus allowing us to escape.
Oh, and Toru, I'm not quite sure where you're going with the strategy of locking the soldiers in the cafeteria. Is it something you think we should aim for right before we engage in the riot, as another form of sabotage? There is little chance we'd ever get all of them at one shot, but it could be another way to slow them down when we're right up to the point of instigating the riot. In the meantime, we're supposed to be locating places like the armory (to perhaps tamper with their weapons) and any place that holds equipment they use to quell insurrections, along with the maximum security wing. So, yeah, is your current idea something you want to do towards the end?