I agree, it is reductionist to claim that WWI happened just because Gavrilo Princip, or that the Kaiser "engineered" the birth of the Soviet Union with the seal

I agree, it is reductionist to claim that WWI happened just because Gavrilo Princip, or that the Kaiser "engineered" the birth of the Soviet Union with the sealed train. In this case the coup was too advanced to be aborted (top generals were already too implicated) so it would have happened one way or another. That the war itself and the dictatorship afterwards could have changed if Franco had to resort to other transport is indeed quite possible, but we will never know.