To summarise the thoughts of those who responded to the OP so far. If I get the essence wrong please feel free to correct by memory is apparently somewhat sieve-like.
At this point only Howard feels that the proposal that the available evidence is not suggestive that the 3 witnesses did not collude in an attempt to protect Williams from the floor seats were associated with. I would point out that the evidence presented was available to the WC. Howard feels that it was simply a matter of some sort of common group mistake......sort of a reverse gorilla experiment effect where somehow a person is included in an event who wasn?t present.
JohnM feels that Williams, who eventually claimed to have gone alone to the 6th floor in anticipation of joining workmates, saw Oswald in the SN and for some reason then decided to leave and look for others on the 5th floor.
The others who have posted are generally in agreement that Norman and Jarman would be inclined to help Williams by lying given the circumstances.