I think they offer rear facing because there is insufficient headroom for forward facing seats there. The kid's heads would be very close to the rear window, ie directly in the impact zone. Also, there is no access to a forward facing 3rd row seat, unless you have the kids climb over the seat.

I don't think having seat belts built in to the seat solves any problem, because what is critical is that the seat belts are suitably anchored. If you are going to build brackets to hold the seats, suitable for crash impact, then those brackets will also anchor the seat belts.

With the E70, it isn't just a bracket bolted on. BMW designed a new vehicle pan to mount the seats. It is impossible to add the seats to an E70 vehicle with the floor pan of a model not designed for them. This became clear when X5M owners asked about adding a third row, and BMW said it wasn't possible because they used two different designs of floor pan, and the X5M was designed on the 2 row floor pan.

If you buy a third party seat from a reputable manufacturer, they will have calculated the loads due to impact. If you take a seat and build brackets yourself, how will you determine how strong the structure is, given that your children are going to be sitting there? That sounds about as safe as home-made airbags.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue