hi there.
the bmw designer did very pretty good job with the seatbelts design.
its just the installation process that is described wrong in their manual.
i almost tore my backseat trying the seatbelt method.. and failed miserably. swearing and hitting things for no apparent reason
its just a backseat in the end.
here's how i did it in 3 minutes flat after that:
I used the torx 50 to remove the seatbelt buckets support from the car.. simplu slide them back in the backseat... then use the available isofix holes to fix them back to the car while the backseat is in the right place..
hoefully this can help other members with similar situation without damaging or forcing anything.