It was an acute pharyngeal infection. Mercury had zilch to do with it.
EVERYBODY with a fever got calomel (mercurous chloride) because it made you salivate and sweat. The old-fashioned doctors that attended GW were trying to get the "humours" out of his body. A younger physician in attendance tried to suggest a tracheotomy (iirc), but was overruled.
He probably would have died anyway because of the massive infection and lack of antibiotics.
I am neither a doctor nor have I played one since I was 3 or 4. I got my information from Thomas Hager’s “Demon Under the Microscope”. He has a master’s degree in medical microbiology and immunology. He stated in the book that GW was treated with massive doses of Mercury and that it may have been responsible for worsening his condition, but you may be right.