Why would a custom order need more capacity? Either a buy a standard model or I buy a custom model. The assembly line is basically the same, just a few different input parts. The assembly line will be rolling out the same number of cars per day. The difference being my custom order got sent to the factory and the next car started on the line was my car.

This has nothing to do with the colour and seats though. When "your" car finally rolls of the line to should take ... what? 1 working day to spray it the colour that you want and let the paint dry, an hour to bolt in your choice of seats and Cd/Mp3 player.

That is not how car production works. Paint is literally the second thing that occurs after a chassis and body panels are applied. But otherwise, the working time on a car from start to finish, including paint, is somewhere in the 10-20 hour range. Theoretically, your timeline could work but paint takes time to cure, then needs polish and interior features take a while, although CAN-BUS has certainly made the electronics easier.

And nobody claimed otherwise. Order backlogs introduce the large delays; manufacturing/shipping takes days or at most two weeks.