Hi, I've been searching over the Internet for this issue for quite some time. Here are some real pics in threads from other forums. Those are E93s though, should apply to F33 which has more back room.
https://www.e90post.com/forums/showthread.php?t=71596
https://www.m3post.com/forums/showthread.php?t=481729
https://www.e90post.com/forums/showthread.php?t=92323
As much as I want my kid to ride the BMW from an infant, I still think it's a bad idea even with a perfectly fit car seat. The rear windshield will shatter by the anti-roll bar in the event of a crash, glass will hit your baby. I got my rear windshield tinted as soon as I got the car, but I still have this fear.