I take it you believe someone "encouraged" Williams to slowly migrate his story to be eating lunch on the 6th floor after noon.
Maybe! Or maybe Mr Williams did spend a v. short time eating his lunch up there on his own, awaiting the expected arrival of his workmates, before being ordered off the floor by... someone.
Initially he tried to hide
any visit to the sixth floor. Then he gave a series of garbled versions of the truth.
They also "encouraged" Norman and Jarman to eventually abandon their belief that Williams went up with them.
They never believed anything of the sort! And most likely it was Mr Williams himself who did the encouraging on the score of backing up his story.
If it was Piper, surely he simply becomes the de facto Williams. Black employee who left the floor before the shooting, "nothing to see here".
I'm not arguing that it
was Mr Piper, just that he fits Mr Rowland's description much better than Mr Williams does.
If not Mr Piper, then a member of the assassination team.