I'm no expert but I'm pretty sure theres no definitive answer to most of your questions. The best thing you can do is to listen to your symptoms and go from there. What triggers them, what your tipping point is, what helps, what doesn't and work on slowly building up your tolerances. That sort of thing. Every concussion is different so no one can tell a definitive answer that applies to your specific situation exactly, you kind of just need to be your own guide and listen to your body.

No one can say how long your recovery will take, its not something you can really control so theres no point in pondering over it.

I have never heard of headache killing brain cells unless its caused by an underlying condition like meningitis or aneurysm or something that would give other clear signs (high fever, confusion, numbness, etc) that something was seriously wrong.

Mark will likely give you a more in depth answer but that's all I got.

Exercise is actually good for your recovery. There are several concussion rehabilitation protocols that specifically include graded return to exercise, working just below the level that triggers adverse symptoms. Best to have a customised program developed by someone like a neuro-physiotherapist or an exercise physiologist with good knowledge of brain injury. My son's neuro-physio also had him doing exercises in a hydrotherapy pool. See what your dizziness will allow you to do and gradually build up. Exercise bikes are a good way to do some cardio. And definitely avoid lifting heavy weights.