But nobody here, not me in particular, said that it was a well known symbol of appeasement in Dallas (or anywhere in the US) at that time. So it's not surprising that only one person used it that way. My posts were in response to the original poster, Michael Griffith, and his claim that it was "absurd" that anyone would use the umbrella as a symbol for anything in protest (as Witt claimed). He said he never heard of its usage for such things.
But the links I provided show the origins of this symbol and how it came to be seen as a sign of appeasement in the UK in particular or weakness politically. In another link there was a story about some German students sending JFK umbrellas as criticism for his lack of response when the Soviets constructed the Berlin Wall.
There was nothing in the links about how popular the idea was and/or whether the Birchers/Klansman in Dallas (or elsewhere) knew that it could be used to heckle JFK because of his father's support for the appeasement policies of Chamberlain. Some people knew about it; but most probably didn't.
Apparently Witt knew something about its symbolism. As he testified:
Mr. GENZMAN. Why were you carrying an umbrella that day?
Mr. WITT. Actually, I was going to use this umbrella to heckle the President's motorcade.
Mr. GENZMAN, How had you gotten this idea?
Mr. WITT. In a coffee break conversation someone had mentioned that the umbrella was a sore spot with the Kennedy family. Being a conservative-type fellow, I sort of placed him in the liberal camp and I was just going to kind of do a little heckling.
We have Witt himself says he vaguely knew about the symbolism and how it was a "sore spot" with the Kennedys. So he decided to just heckle him with it. We have one person doing an odd thing. And LBJ knew about the symbolism too when in the campaign he directed a criticism at Kennedy Sr.
Y'know, not everything has to be jammed into the conspiracy against JFK? Sometimes a guy scratching his nose in a photo in Dealey Plaza is not a signal, it's just a guy with an itchy nose?
But nobody here, not me in particular, said that it was a well known symbol of appeasement in Dallas (or anywhere in the US) at that time. So it's not surprising that only one person used it that way.This appears contradict other your other posts.
Whatever the case, you now seem to agree that the umbrella was a weird and unusual thing to do which is evidenced by the fact that one single person decided to do it. Witt knew nothing of the Kennedy/umbrella connection, he just overheard someone mention it and that was good enough for him.
What makes Witt suspicious is not his use of the umbrella (that's more odd than suspicious), it's his testimony. Witt's HSCA testimony is hard to swallow. In it he states that as the motorcade is coming down Elm he is sat on the grass of the grassy knoll. He stands up, begins to walk forward whilst opening up his umbrella. As he is opening his umbrella he hears three or more shots (but doesn't recognise them as shot sat the time), and misses what is going on because he still hasn't opened his umbrella. By the time he gets his umbrella open he is aware of the limo slowing down, a Secret Service agent running towards the limo and "a pink movement...Jackie Kennedy, I think, wearing a pink dress or something."
However, Willis 5, thought to represent Zapruder frame 202, shows the umbrella clearly raised. This is way before the throat shot or the head shot:

Betzner 3 (z186) shows the umbrella already up in place even earlier. It's partially obscured but it is picked out by the red arrow below:

Witt goes on to state he never saw JFK hit, was unaware he'd been shot and was only aware that there had been slowing down of the limo and Hill running towards it. Yet he was aware "something terrible had happened" and was so stunned by what he'd
not seen he had to sit down.
Witt claims to remember the limo slowing down and Hill running from one car to the other. This is the moment of the head shot, the moment JFK's head explodes yet Witt seems to have missed this detail. Strange, considering he'd made the effort to go out of his way to heckle JFK specifically.
The problem is that Willis 5 and Betzner 3 show UM already in position, umbrella raised and, I would argue, before a single shot has been fired.
His testimony appears to be a fabrication - this is what makes him suspicious.