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.