It takes a lot as compared to brow treatment. 15 to 20 units per side. Too much can interfere with chewing.
The important thing is for the follicle, the structure that makes the hair, to survive. Sometimes the the little hair in the follicle just starts growing, sometimes it falls out, but as long as the follicle is OK, it will make a new hair. We suggest that our patients use Rogaine before and after surgery. It may help the transplanted hair to keep growing rather than fall out.
I believe that it is remotely possible to cause permanent motor nerve damage with an injection needle by physically lacerating the nerve. The nerve which controls the muscle that keeps the brow up is, however, not in the path of a Botox treatment, so it is very unlikely that a Botox treatment can cause irreversible brow droop.
Your hair loss looks like it is genetic, but a dermatologist's evaluation with a biopsy, if necessary, should be considered before doing anything. I believe it would be a bad idea to have a hair transplant, especially for the back at your age. It would be best to try to stabilize the hair loss with medications first. Good quality professional photography can help to determine if the hair loss has stopped, or at least been significantly been retarded. Wait a minimum of one year of stability before considering surgery. Save your (limited) donor hair for the front.
Botox "wears off" gradually; it's not working one day and gone the next. Usually at four weeks post treatment this process loss of treatment effect has not begun. That said, I have had a couple male body builders who were quite resistant to Botox. They required more and did not last long. The usual explanation is anti-Botox antibodies. I've wondered if something about this activity, bodybuilding, has an effect on all muscles including the facial muscles even though they are not the ones being intentionally exercised.