I doubt anyone will exit just to get pole for the second race: It's not that sort of series. Saying that, if it did happen, I think the solution you have will do the job very nicely. You could even have a designated 'pull over' point for any perpetrator(s). If it could be proved that the person exited just to get pole, perhaps a drive through penalty would be a better penalty?
As to starting the race immediately, there will always be exceptions, but common sense should prevent these from becoming problems: A late arrival may miss qualifying and complete the first race but then need to change transmission or some other settings. Common sense would dictate that they should be given a minute or two to complete these changes.
There are also those with weak bladders to consider...