I would add that any planner would have to know about the open car. Was Oswald or others just lucky. In killing Kennedy I think they said Kennedy told the ss the day before, no top. How could anyone rely on that?
No, the decisionto leave the bubble top off the vehicle was made at the last minute when the skies cleared sufficiently. Kennedy was there on a political trip to firm up his standing in Texas and wanted to portray as much openness as possible to the onlookers.