If the BMW dealership says you need transmission parts which aren’t available, not much else you can do but wait for the parts. Your solution to rent a car in the meantime makes complete sense. Be sure to keep dated meticulous paper records of all your expenses (including car rental) , as well as of all conversations you’ve had with BMW and the dealership. The best way to keep a record of an oral conversation is to make notes & send a letter to the person you talked to, asking them to verify your account of the conversation is accurate. In the way of misery loves company, you may be interested in the story of Car Talk family’s Alex Magliozzi and his BMW parts-not-available problem.