In case someone finds this thread later and is interested in the "resolution": I was able to get in touch with someone at the TÜV and clarify the matter. Contrary to the original statement from the window manufacturer, no fall protection 90cm above the top of the window sill is required. Instead, the total height of the fall protection must be at least 90cm and the protection from the window sash must be at least 60cm. Thus, the total height of the fall protection at about 105cm above finished floor level is still slightly raised, but nowhere near as bad as originally feared (>125cm) and claimed by the window manufacturer.
Therefore, we will keep the parapet height 50 in all rooms to keep it uniform. The 105cm fall height is okay for us, it is similar to a parapet, e.g., on a stair railing.
Note: The above statements only apply to Bavaria and only for fall heights <12m and only if the window sill does not have an "over-depth" (window seat).
