Don't worry much. If you buy a Tesla this year, you will get 100% of the $7500, even if it is in December.
The tax credit "starts to diminish" after two quarters of 100% tax credit *after* the 200,000 car is sold. The fact that it won't "run out" until 2020 is the point.
Sell it July 1st and you are going to see:
Q1 100% 2018
Q2 100%
Q3 100% (sell #200,000 on July 5th)
Q4 100%
Q1 50% 2019
Q2 50%
Q3 25%
Q4 25%
What's the fuss? The goal would be to have the factory "ready to crank up" some time in June for "what could be an infinite amount of output" in H2. If they make 100,000 or 200,000 cars in H2 of 2018, all USA buyers could basically get $7500. Timing is everything.
The Solar PV credit was extended by 5 years for 2017-2021. In 2016, before extension, there was a lot of Solar PV work out there, lots of projects. The solar PV installations for 2017 dropped even after the credit was extended due to pulling projects into 2016. if the push is to do a lot of cars in H2 of 2018, it may have a deteriorating effect on car-sales in 2019 as the lower priced Model 3s will be going out and have less support of the tax credit.
Now, if for some reason, #200,000 is sold in Q2, it looks like this
Q1 100% 2018
Q2 100% (sell #200,000 on June 29th)
Q3 100%
Q4 50%
Q1 50% 2019
Q2 25%
Q3 25%
Q4 00%
The problem here is how well will people properly handle the "date of service" of the new car? Will they all clamor into Q3 or cheat on taxes and claim 9/30 is their car's first date of service? I think it is more prudent to try to let the slide happen in Q1 and not Q4. We do know a lot of people claimed their got their cars in end of 2017 even though delivery didn't happen until January 2018. For the tax credit.