The Lincoln assassination is pretty well-publicized with the Civil War backdrop, but the Garfield and McKinley situations seem to be considered basic (for lack of a better word) political attacks by mentally unstable men (Guiteau and Czolgosz).
-Did any of these three tragedies draw any sort of doubt or accusation of "alternate" events, such as the Grassy Knoll or Magic Bullet? If so, was it from civilians, politicians, both?
-Alternately, did any of the "natural deaths" of presidents in office (W.H. Harrison, Z. Taylor, etc.) spark any substantial suspicion of foul play?
Apologies in advance if I still mucked up any of the rules/popular questions.
You are right in your description: the McKinley and Garfield assassinations were bold, broad daylight shootings where the perpetrator was clearly and immediately recognized. Their motivations were also quite clear (Guiteau wanted a job, Czolgosz was an incensed anarchist). Lincoln's assassin also had an obvious motive owing to the Civil War.
As for the other natural deaths, William Henry Harrison died of pneumonia. It was very likely from him delivering what was the longest inaugural speech ever in the freezing rain, and he died 30 days later after being sick for many of them. Not much speculation there.
Zachary Taylor is an interesting one. He died after about 2 years (if I remember correctly) in office, when he was celebrating Independence Day in DC and famously consumed a ton of cherries and milk and died a short time later. Though it was ruled to be a stomach illness, rumors of poisoning abounded for over 100 years. Scientists finally exhumed and tested his bones in the 1990s - he wasn't poisoned. It was most likely malaria of some kind, brought about by poor sanitation standards coupled with DC's muggy, hot summers (remember DC was a swamp for a long time).
Warren G. Harding died of a stroke and no foul play has been given any serious scholarly credibility.