The whole grid penalty thing was so easy to solve I can't believe it never came up. All they had to do was award a nominal time per slot on the grid, lets say 1 second, then when a car gets a grid drop of 10 places but can only take 3 of them before it's at the back of the grid the car starts from the pit lane and is held for 7 seconds after the last car has passed. (Alternatives are stopping the car for 7 seconds at the first pit stop or adding 7 seconds to the cars time at the end of the race). That way the penalty is done and dusted in a single race.