Two points:
1. In their book Beyond the Fence Line: The Eyewitness Account of Ed Hoffman and the Murder of President John F. Kennedy, Quinlan and Edwards thoroughly address the criticisms regarding Hoffman's location, his distance from the area behind the picket fence, his view of the area behind the picket fence, and the plausibility of seeing events/people from those distances (pp. 63-104; pp. 76-117 in the Kindle edition).
Quinlan and Edwards spend considerable time on Posner's attack on Hoffman. They show that Posner's assault is misleading and false.
2. Quinlan and Edwards debunk the claim that Hoffman "changed and added to his story over the years." He did not. There were some issues with some of the ASL translations of his sign language, and the FBI misrepresented several aspects of Hoffman's account. The record shows that Hoffman was consistent over the years.