Not necessary, because even if some mercury could be eliminated, that doesn’t change the fact that IF we find mercury in Beethoven’s hair, we might be able to draw conclusions from it. The author never suggested that we will find mercury or that we will find all of the mercury that was ever in Beethoven’s body.

Necessary, because if were not true — if EVERYONE in Beethoven’s time DID ingest mercury — then the presence of mercury in Beethoven’s body wouldn’t signify any connection to venereal disease.

Not necessary, because mercury can be indicative of the fact one needed treatment for venereal disease even if it wasn’t effective treatment. People could have taken mercury thinking it was effective, regardless of whether it actually was.

Not necessary, because the hypothesis at issue is whether venereal disease caused Beethoven’s deafness. The author never suggests that the mercury itself caused deafness.

Not necessary, because the argument isn’t based on any comparison of psychological problems between Beethoven and Newton. It’s not even clear Beethoven had psychological problems.