The cars are being continually built and order configurations are sizing batches. If you delay your order then that car you ordered is just being sold to someone else who also asked for that configuration. After the car is scheduled for its first delivery, it may sit up to 3 days after that day. If the buyer who matched it passes on it then it will be assigned to the next build match in the queue. Theoretically a specific car could continually sit 3 days each time indefinitely, but statistically someone will want to drive it home. Based on no data whatsoever, I’d guess with the high demand, most cars aren’t sitting past more than 1 or 2 buyers if that.
As far as ordering FSD, yes you lock in that price, but if you don’t want it anymore then you may forfeit your order fee of currently $100 to change your order and also reset your wait time. If you end up not wanting it at the very end, maybe they won’t care because it’s just a software change and maybe you have to pay $100 to change the order. I’m not entirely sure if they will put you in the back of the queue or not because ultimately someone will have that build matching an order somewhere else in America, so they can sell it to someone who has also been waiting.