If they tried to start the war without him he would most likely blacklist them, sever all ties, and have his Ex Jedi Master turned Sith Lapdog, Dooku to mop up, meaning DECAPTIATION!!!!!...via Lightsaber so they don't squeal. He would then place someone more competent to his plans in charge, likely the second in command or another like minded indivual or group that he considered hiring for the plan, but instead chose the ones he did choose. A good Sith Lord always has a backup plan. Palpatine is no exception.
True. Palps probably wouldn't let things happen on their own. This is all speculation of course. My dad actually asked me what would happen so I wanted to get some input from people more learned than I am.