Is a higher knee wall permissible for approval as a result of the construction acceleration § 246e of the Building Code?

In the end, effectively only 60cm remain on the interior wall when you subtract screed and flooring, or am I seeing this wrong?

That is at least a perfectly complete vacuum knee wall, all good. Then you simply cannot push a bed all the way to the wall on the eaves side.

, or am I seeing that wrong?

Yes. You did not understand my sketch and my proposal.

The blue lines indicate a height of approximately 1 m. In my opinion, the 80 cm can also be measured from the top edge of the finished floor (OK FFB), since the BBP says nothing about it.

The purlin can also be moved to the outer edge, adding a few more centimeters.