I don't know if a cracked manifold is common or not, but the one on my 97 was cracked. It didn't cause the problems that you explain, but it also wasn't cracked as bad as your in that I couldn't hear any exhaust leak at all. But it did run better when I replaced it. Your best bet is to start with replacing the things that you know are wrong. Start with replacing the front manifold and go from there.
It is very hard to diagnose a car when there is something that is bad. The cracked manifold can cause all kinds of problem if bad enough. And it is pretty bad if you can hear the leak. And if you can hear air coming out, then outside air is going into the exhaust, and messing up the reading of the O2 sensor. So change what you know is bad, the manifold and see what happens.
As for the MAP sensor. Yes, these cars have a MAP sensor. One thing though is that you never said what model of grand prix it is. Is it a SE, GT or GTP?
As for the cat, I wouldn't trust a cat that is over 130k miles or so. Sure some people run them for longer. But they get hot and wear out. If it is possible, I would just remove the entire cat (check local regulations first). If not, get a good high flow cat and a larger downpipe, and get rid of the very restrictive u-bend. All of that can be done considerably cheap.
