I was allowed to defer my order for 6 months max. I ordered right at the end of September 17 and took delivery mid Feb 18. You can't nail down a specific delivery date, but should be reasonably close. So if you want an early March delivery, you'll probably have to wait until October to order. When they do arrive they push to register them immediately, so I wouldn't count on them holding it back for you.
Seriously don't waste your money on FSD. Your couple of grand isn't going to make the slightest bit of difference to its development and if you have been paying attention to the current state of EAP, you will realise that FSD is a million miles away from any sort of reality. At best you might get some token gesture features like traffic light recognition. But it's your cash to burn. Plenty of others have been suckered into it over the last few years!
Free supercharging is nice to have, but only of significant benefit if you are using it a lot. You get a free 400 kWh p.a. allowance anyway, which is more than enough for my needs, although I did get the free referral supercharging + £750 discount at the time.