There are many different types of child seat. ISO Fix seats aren't just for babies they're for young children too, who require the seat but use the seatbelt to secure themselves in. Modern ones have chest protectors which velcro around the seat belt to protect the children's chests.Are you sure you aren't going a bit OTT here? The point of ISOFIX is you do not need the seat belt. The seat belt attachment is for folk who do not have ISOFIX.
Thank you this is a potential solution I guess, the cabin should be large enough for someone to climb over that into the back. I wonder if the X5 is similar?
Thank you so much I appreciate the effort.