The issue with the 12/17 SUV is that rates go up on the 18th and even if you say you'll pick up at a certain time the contract won't officially start until your BIL picks up the car. There is a no chance that he will make it to the car by midnight if he gets in at 11:40. If you put his flight info into the reservation they
might honor the rate even if he picks up after midnight but he won't know until he picks up the car.
Did they tell you you could be added as a driver? I believe that with
Alamo you need to add the extra driver when you pick up the car *and* they charge $10 per day so that increases your rate by $100. There is also a very slim chance they will change the credit card after the car is picked up.
I would go ahead and book the other two while you still can. You can always cancel later. Frankly, I have not had a problem with
Budget or Dollar and would not hesitate to use them again.