1. Don't think of it as football teams all running the same offense and defense. Think of it like all football teams having to wear the same uniforms (shirt, pants, shoes, pads, helmets, etc.) and having to have a field of the same dimensions. However, just like a team might prefer one helmet over another, or have different logos, or different types of grass for the field, NASCAR cars have MANY different setups. Setups involve a huge number of things including, but certainly not limited to, tire pressure, gear ratio, spoiler angle, springs, spring wedge, grill tape, front-rear weight balance, front-rear brake, and on and on and on, and I didn't even get into the engine yet.
A lot of people who don't follow NASCAR think that all the cars are the same, but it's far from the truth. NASCAR simply has a set of templates that the car must fit into. But those templates are not a strict measurement. They're all a range, meaning one Chevy can be completely different from another Chevy and even more different than a Dodge. Now to you and me, they all look the same. But if we were to drive them at 150+ MPH, I'm sure we could tell a difference. Everything in NASCAR is about ranges. It's what you do within those ranges that makes all the difference. If you want to get a good sense of this (about as good as any normal, non-driver can get), buy a NASCAR game like the one from the folks at Papyrus Racing and make sure you get a steering wheel game controller and set the game to simulation mode (not arcade mode) and have fun. Adjusting car settings even the most minute amount will have dramatic affects on your car.
2. They don't. I can go upwards of 6+ hours without peeing if I have to (the key there is "if I have to"). My parents used to take me and my brothers on vacation cross-country and there wasn't much time to stop for peeing, so there wasn't much peeing. When you're driving 150+ MPH, I think peeing is the last thing on your mind anyhow. Just go before you get in the car and you can make it.
3. They do try to win. But again, running 150+ MPH, you don't want to do anything to jeopardize your teammates. In other words, both cars finish 1st and 2nd, it's better than the 2nd place car racing for the lead at the end and causing a crash that could send one, or both, cars further back. A good example of this is what happened to Jimmie Johnson last week. Granted, Sterling Marlin isn't his teammate, but something like that could easily happen to teammates just by accident.
Plus, at some tracks, like Daytona and Talladega in particular, if you try to pass without "passing buddies" (to keep the draft going), you're going to get hung out to dry and go from racing for first to finishing in 30th, less than a second behind the leader. Take a look at the Pepsi 400 results from last year. Note that Dale, Jr. finished 6th. He was in 2nd on the last lap and tried to go for the win. Nobody went with him and he got hung out. This cost him money, finishing spots, but most importantly, it cost him points. Had he finished in 2nd, Jr. would have gotten 20 extra points. 20 points here and 20 points there add up at the end of the season. Jr. missed the top 10 in the overall standings by just 53 points. That's the difference between getting on stage at the Champions dinner (what all drivers shoot for after shooting for the championship) and not.
However, mostly the team concept is about sharing information. You know that your teammates are running very similar setups/car bodies to yours, so if they tweak a setting and it helps their car, it's likely to help your car. Much like having coordinators in football, the more people with input, the more information that's available, the better the decisions likely are.