Here is DIY for removing the rear seats on 5er F10 without through loading option (465)
There are different possible reasons that you may need to remove the back seats, e.g. Having access to parcel shelf to add speakers or subwoofers.
Here are the steps:
1- remove the bottom part of the seat by pulling its front up. There are two notches (shown in pics) that are holding it in place each one approximately at the center of each of the back seats. Pull the seat up at those locations.
2- push the seatbelt locks into their hole.
3- slide the seat toward the front of the car and up.
Note: you may need to do steps 2 and 3 in intermittent efforts to release the seatbelt locks.
4- using a Torx 45 unscrew two screws at the bottom of the back part near seatbelt locks (shown in pics)
5- at the two lower corners of the back part, near doors, there are two long tongues. Slide them out of their hole by pulling the corners up and front.
Note: these tongues may pop back into their holes.
6- pull the seatbelts and put the behind the edges of the back part (shown in pics). If you don't do this step when you are pulling the back part explained in step 7, the seatbelt auto locks won't let you do your job, so you have to keep them away.
7- pull (hard) the back part up. There are two notches (shown in pics) that the back part is locked in them and by pulling up you should release the back, so don't be afraid to pull hard.
That's it.
When putting back everything go to the reverse order of the steps above. The most difficult part of putting back is when you want to put back the seatbelt locks into their holes in the seat while sliding the seat back into its place. (good luck with that)
UPDATE: for the seat belt problem, you either can use lancar's shoelace idea or can use duct tape (sorry for poor mobile quality):