I must say I was surprised when I ordered my TM3 SR+ that I could not order the white interior. Seriously, full glass roof in Australia, I think they are mad! Or else they are trying to upsell to the AWD models, which is just not feasible for a lot of people. Personally I know three people who really stretched to buy their TM3 SR+, no way would they go the AWD for over $20k more. Plus add the cost of white, plus add the cost of a colour (red just went up from AUD$2,500 to AUD$2,900 in the week I ordered mine). It all adds up and it gets pretty expensive (and they don't even put a Frunk liner in, and they don't provide the wall charger, so there are added costs most people will fork over because they are pretty essential).
Tesla even promotes the TM3 as this light an airy space to be, but then dulls it down with black seats. After having a light interior vehicle for several years it will be very hard to go back to black. I've looked at white seat covers, and that will probably be what I do, but the rear seats are just not done very well when you consider Tesla could have offered white in the first place. The other option will be to see if there is any way to order spare parts, eg: from a wrecking yard, and get white seats that way. More a long term upgrade and no doubt would cost more than the $1,500 it would have cost to just be able to order them in the first place.
As to the order process, it is all on-line. I've worked in manufacturing and this order would be sequenced into a batch with all the others of the same type, in fact seats would be the easiest one as they are just a picking item that gets slotted into the production sequence at the right time. It's all robotics and computer scheduling (if it's not then I would be incredibly amazed). On the Tesla website, when you order, it just tells you it could be 12 weeks for delivery. The order goes straight to Freemont and your car arrives whenever it gets here. Sure, you can buy the standard model that is on a ship today, and get it really quickly, or you can buy a custom model that really takes no more effort from Tesla, and get it in a few months. I bet many would be willing to wait.