1. First of all no such thing as an 80 gallon tank. Tanks are rated in cu ft or liters. As for how far they can go on one tank, it really depends on what you consider the "average diver" to be. Average as in warm water vacation diver? Regular diver in local cold water conditions? Average technical diver? You'd need to have some idea of their consumption rate. How realistic do you want to be and who is your target audience? This is a problem I run into when writing. Am I writing for new divers, non divers, experienced divers, or tech divers? Non divers you can do what movies like The Cave and Sanctum did and just make up crap. Facts are not as much important as they are boring and annoying. For divers if you go into the detail you need to in order to educate non divers it can get boring.
Why do they need to get through this and did they plan for it? Are these dry cavers that sometimes use scuba for sumps or actual cave trained divers? I know some pretty accomplished dry cavers. One I went to school with. For his teams if they came to such a point they would call in another team with proper training. Or look for an alternate route since none of them are divers.
2. As long as they are under ground they will not suffer from DCS. It's how long they have been down, when they come up, how fast they come up, etc. that determines the risk of DCS. 200 meters is a bit over 600 feet. The air is however not compressed as if they were working in a caisson. Coal miners regularly spend 8-12 hour shifts much deeper without suffering any decompression related problems on ascent because the air is not compressed. A five meter depth dive 600 feet underground is going to be about the same as a 5 meter dive in a lake.
3. The problem here is how are the transporting the cylinders, how are they going to carry them through the sump (back mount, sidemount, slung, dragged along), and have they done the planning and gas matching to know that this will indeed be enough air? How will they move the cylinders to do the switch. What lights are they using to allow them to see to do this? How do they know they can even get through the "tunnel"? What if they get in far enough to use up two cylinders and then find they can't get any further? They have used two to get in, get stuck or stopped, they have one left which means the story ends quickly as they all drown or they sacrifice one or two and hope they have enough to get back out.
Not trying to be an ass about it, but one thing I have learned from books, movies, and writing is that if you really want to do a story justice, you need to be personally familiar with the subject. Otherwise it can come across very poorly. Any work is going to have it's critics. No matter how good it is. But not knowing your subject and trying to write about it with any number of people who do know the material as part of your target audience is going to seriously detract from the story.
Unless something of real significance is going to happen in that tunnel, I'd look at ways to eliminate the scuba aspect of it. Unless you don't have divers as your target audience.
