They have been setting the pieces to this eventuality for 60+ years.

Trump just stumbled onto the very carefully set board, and started messing with it, and exposing the plans in the process. Being the narcissist he is, he is incapable of not using anything he wants, so it forced the Republican establishment’s hand. They had to bring him into since the Presidency is necessary to further the plan, and in turn he also became dangerous since he’s 100% the type of person to use that information to extort the outcome that helps him. I don’t think they really understood how bad an idea that was at the times and now they’re stuck.