Sorry to hear about you luck with the car. Some cars get built better than others, it's sort of a numbers game.
The air conditioner not working in any mode but full is a problem with the A/C Resistor pack, when the fan is in FULL mode, it by-passes this device to allow the full twelve volts to make it to the blower motor (which generates that wonderful gust of air). This part is also called the Blower Motor Resistor. The part shouldn't be too expensive, and replacing it should take 1-2 hours. I can't say for sure where it is on your year specifically, but it is generally placed by the blower motor, under the glove box. It will have about 5 or 6 wires going to it.
Nissans are somewhat notorious for less than great electrical systems, especially for wiring, and I've had my share of the problem. The strange noise you might be getting from the car when you attempt to jump it may be a very weak electric current running through the relays. In that case the problem tends to be a dead cell in the battery. When this happens the battery has a short in it and it absorbs all the juice from then car, hence jumping will not work. (This has happened to me). One way to check is to activate the Hazard Lamps (with the key off), if they dont come on, or create an interesting buzzing noise, the battery or it's connections are probably the problem.
I'd like to offer more help but I'd need to know what sensors are considered the culprit for the Check Engine lights illumination. Also, if you've replaced the same Oxygen sensor twice, there may be a fuel problem, either with a leaking injector, or with bad gasoline. Fuel injectors are actually pretty easy to replace on the Altima 4 cylinders. (KA24DE) The injector could be dumping too much fuel into a cylinder, and then setting the Oxygen sensor as failing in the Computer. This would definitely cause the Check Engine light to come on. But this is a vague guess to their premature failure.
Also, your car doesn't have a timing belt, it has a timing chain, which is very long lasting, generally 300,000 miles and then some, especially when she's getting a regular oil change.