As others have pointed out, you are wrong concerning the refundability of the $7500 tax credit. You are correct that it shouldn't be used for monthly payment calculations as the buyer will still have to pay full price and then apply for the refund when they do their taxes in the following year.
When I did calculations for my monthly payment, I did it without subtracting the $7500 from the originaly price. I also did a "what if" scenario to see how much my payments would go down if I refinanced my load after receiving the state and federal tax refunds.
Btw, my federal tax refund was $7500
higher and my state tax refund was $6000 higher thanks to my Tesla purchase.