Booking.com forcing the account holder as the primary driver no matter what is wild. Listing a nickname as the driver and then charging extra every day just to fix it feels like a straight up gotcha fee.
The under-25 situation with your friends is even worse. Making a 20 year old the primary driver for two cars at the same time makes zero sense, and the fact they only refunded one car says a lot.
This really does sound like bad programming mixed with zero accountability. I’ve used Booking.com plenty for hotels, but this is enough to make me avoid them for rental cars altogether.