“Calories after exercise?”

I've heard that women need to eat at least 1200 calories a day. I've heard that to lose weight you have to burn more calories than you eat, but I've also heard of a female athlete triad which is bad because you're consuming fewer calories than using.

1. Does that mean that you need to have a positive intake of 1200 calories after you've factored in exercise?

2. I've heard that people on drugs that (accidentally) suppress appetite can end up in the ER, but then anorexics go years without ending up in the hospital. And supposedly it's safe to lose up to 4 lbs (2% of body weight) per week, but to lose 1 lb per week my net intake would be less than 1200. I don't even know what question to ask here.

3. How long does it take to be below 1200 before people end up with medical issues? I've heard that you should eat like 800 800 800 2400 to make sure your metabolism doesn't get affected. Mathematically that averages to 1200 a day. Or will that put you in the hospital?