I can see your point of it too. The main difference is on the right side you have this one "30%" seat moving totally independent of the other 70%. The fact there are 3 individual backrests isn't as important IMHO. If you had the whole rear seat set removed, maybe it would be easy to see why I would look at it that way (HERE'S the 30, THERE'S the 70
).
There may be a way to add LATCH that I am not aware of, it may be a good idea to contact Phil or an MB dealership to see if they show any LATCH components that you may be able to adapt to your rear seat(s). They are built into the rear of the seat frame, but I'm worried there is extra gussetting on the newer frames to bolt these to, if they do bolt, or worse that they are welded on, in which case you need a seat frame or two (big bucks, and HOPE they will bolt to the floor. OH and you lose the individual backrests too. In short, probably a no-go. Only hope there would be to source out a rear seat (and still hope it bolts down). I also don't know if it would be that good of an idea to go "reengineering" the seat to accept LATCH, likelyhood of a catasrophe is slim, but exists.
ONE thing you can research, is found on page 47 of the 98 ML owners manual which discusses a "top tether strap" anchorages for Canadian vehicles. I don't know if this is something on the seat, more than likely for the floor behind the seat, that a tether can attach to? May make things safer. If there are reinforcements on the floor pan for these to attach to, I imagine they could be added to a US version with a little work.
Gilly