As explained top 30 get the offer to submit a livery, if they dont want to that is fine, they can race in the car they qualified in.
Grid is set per prequal, as rules explain.
5. Qualifying will be held at set times before each race and will determine which “split” a driver will compete in and the starting order of the race will be determined by this
It is not required, you wont be able to change anything, the livery we get from drivers will be a manual process, we simply change the car they are signed up with to this new car.
So if you do not submit, no worries you simply race in the car you qualified in.