The good news is that Amherst wants you – but it seems that they want to give you every opportunity to be strong and healthy enough to take advantage of the opportunities you will have there. Focus on healing and getting strong. Find something to do that you feel will be interesting and valuable. You have been admitted so you don’t have find gap year activities to impress anyone but yourself.
If you want to take a class or two I would check with Amherst admissions and possibly also the registrar first to be sure that it is OK and that you will get credit for the class.
Thanks for sharing your story and best of luck as you move forward.