May I be the first to say
I don't have an answer for the first and last question, as I'm still learning so much
As for the second, I've consistently heard that a 1-2lb weight loss per week is a healthy aim and anything beyond that will be counterproductive. So if you eat too little and lose more than 2lbs as a result of that, you'll probably be losing a lot of muscle and bone along with some fat - but this is not good for so many reasons... And if you try to exercise off more than 2 lbs per week, I think there's something written about that here. Here is a link to another thread from the exercise forum. I think it's the last myth about "the more exercise the better" that may apply to your question: Top Ten Exercise Myths
By the way, the Exercise forum has a few "Sticky's" (threads at the top) that I found especially useful when I first came here and wasn't sure how to go about exercise.
I hope that helps
Good luck! If you have any questions don't hesitate to ask, someone around will probably have an answer or be able to guide you to it!
And kudos to you for taking these first steps!!