Solved

Driver change during competition

Yes it is allowed to change driver. Regarding championship points:
Team Championship - Any points the driver has won before you make them inactive remain with the team regardless of what you do with the driver.
Drivers Championship - If you keep the driver as a reserve they will retain their points and will continue to appear in the drivers championship table. If you sell them they disappear from the drivers championship.
This is why you sometimes see an apparent bug where a team has more points than the sum of the individual drivers. It's because a driver who has won points is swapped out mid season and then sold.
I hope that makes sense.