I would assume lone TUMs would not raise their umbrellas in the crowds along Main. They might encounter some opposition' or be taken for weirdos. As it was, Witt was standing back from the sparse crowd, out of the sightline of those nearest him.
There was a mention on a Dallas blog (I have no link) that men carrying umbrellas were spotted at the Trade Mart. This would be a good area for multiple UMs to open their umbrellas in protest when the motorcade arrived. Maybe there was something about Love Field that didn't suit them.
Several white-power members known locally were arrested at the Trade Mart (it took the assassination for that to happen). The Umbrella protest was going to be a thing (like MAGA hats in 2016) for Republican Goldwaterites in 1964. The protest was just getting started in late-1963.
Witt said he was in the wrong place at the wrong time. It happened to JFK and the PT109.
Witt testified that someone at work (he was a supervisor at a local insurance company) told him that people in Tucson or Phoenix had displayed umbrellas at members of the Kennedy family at an airport and they, the Kennedys, got upset. So apparently among some Kennedy opponents/haters/critics it was a "thing" to do. Or consider. The idea that the display of umbrellas as protest was unheard of is just flat out wrong. It was done.
Recall that the liberal wing of the Democratic Party, most notably Eleanor Roosevelt, came out strongly against Kennedy's nomination. They thought he was too young and were upset at his work with McCarthy but were particularly worried about the influence of the father, a man who was viewed very critically over his support of appeasement. It's interesting that JFK is viewed as a great liberal hero when in reality liberals at the time were suspicious about him and the influence of Kennedy Sr.
It's also interesting that the chief author of the Warren Report, Norman Redlich, represented clients called before McCarthy's committee. I can't find evidence of it but I wouldn't be surprised to see that there were some terse exchanges between JFK and Redlich over questioning of his clients. Anyone who thinks Redlch would coverup for a right wing murder of JFK is thinking some foolish things.