Not to add to your stress, but I assume your medical clearance was issued by a German authority, correct? If you are permitted to board the flight to Germany, you will still need to present yourself to the Bundespolizei to clear immigration formalities. If you have been issued with a clearance to enter Germany from a competent German authority (for example the embassy, your German doctor stressing the importance of the procedure which was booked before Covid-19, etc) then you will be ok. I think a note from your US doctor, on the other hand, would not be sufficient reason to let you enter. I would call the Bundespolizei and inform myself how this is likely to play out. Good luck!