Schroth Rallye 3 ASM harness installed:
I've chosen the to use the isofix mounting for the rear strap. I know it might not have the same strength as the official seatbelt mounts. But I feel confident it is sufficient. I have from reliable source (seatbelt manufacturer, whose name I wont mention for legal reasons), that todays requirement for official mounts are way overkill. Or rather, the required strength of seatbelt mounts far surpasses anything the human body can withstand. In other words, if you crash hard enough to break the isofix mount, you are with all likelihood dead in any case.
P.s.
Original first post in the thread updated.