This is similar to an academic concern that I've been wrestling with in my mind regarding my own personal situation. Maybe those who are more tax savvy than me can sort this out.
Being retired I'm precisely in the situation that you describe, that is I do not have enough income to generate a $7,500 tax liability. So inorder to take advantage of the full tax credit I have to generate additional tax liabilities. I can do this by withdrawing from my 401K. By withdrawing the balance owed on my Model S, plus an additional allowance to deal with the increased tax burden of moving into a higher tax bracket generated by the additional income, I can create the necessary tax liability to take the full credit. So the question to the tax experts is, "Have I really saved anything by taking advantage of the full tax credit if I have to increase my tax liability to get it?"
In your situation the inverse question can be asked, "Will I really lose money by not being able to take the full tax credit if I move into a lower tax bracket and reduce my overall tax liability?"
Larry