if you miss a chance to unlock a car do you get another chance at another time ?
Nobody knows the answer to that except the developers. Good luck getting them to answer.
Also, if you don’t have the money to purchase the discounted car, press b to skip the reward. You can purchase it later, under progress tab in the menus.
Yes, you do.
I have seen the same alfa Romeo and mustang cobra come and go a few times each, so I’m guessing they will show up again.