AFAIK Mozart died from Syphillis (and Absinthe), just like any other artist in that age, but as he had an umarked, his remains were never found and nothing can be proven.
But you can't really do that in a 1980s Hollywood movie.
Also I've always wondered why the german arias in "Amadeus" were in english. Was that a director's twist to make them more accessible or the usual anti-german bias?
It surely was weird since in the german version, the singers in the Zauberflöte still sing english.
Actually it seems he was not that poor as suggested in the movie. In 1791 there were laws in force in austria, which actually governed that everybody except the extremely famous and rich had to be interred in unmarked graves and there were only set times.
He had huge debts, but did not live in poverty.
Mercury at the time was considered the cure for Syphilis. Alledgedly Frederick the Great was cured to impotence by Mercury, but at least he lived.