Hi,
I had a look at my stripped out car, and compared 3 points
1) The point where the centre dual seat belt mechanism is bolted down
2) The points where the child-restraint points are bolted down
3) The point Ken Henry is using, which is the original point that fixes the rear seats upright.
Points 2) and 3) look identical. Both use two bolts of the same size, with a similar depth of thread and similar base plate
Point 1) uses a single, thicker bolt, with a longer thread. However, this one bolt is holding down 2 seat belt inserts, potentially up to a 200kg load with 2 large adults. Interestingly, a standard insert point from a Sabelt harness will go straight into this thread, and I use this to restrain some gear in my car.
My 2 cents worth about these various insert points.
From a load point of view, I suspect Ken's insert point is similar to the child restraint points, in each, the two harnesses are held by 4 bolts in total of a similar size and insertion. Ken's method is much better in terms of angles, reducing the risk of a seat and spine crush under load

I don't know the true load rating for either point. However, knowing BMW I suspect they are very over-engineered.
My advice would be to get proper CAMS approved insert points, with appropriate backing plates. It would not be overly hard to do to get these fitted to either the child restraint or seat restraint points. However, If you do use the standard points then wear the seat belt on top of the harness. That way, in the worst case scenario that one or both mounting points fail, you still have the original belt to save your upper body.
Cheers
Robbo