What a tether does is reduces head excursion from 32" to 28" (with 0" being the middle of the backseat as it stands vertically). Is it important? Yes. Is it going to kill your child to not have one? No. It's required with LATCH, it's required on some seats over certain weights or with certain installs (and I can count those special seats on one hand. Two if you head into special needs seats), otherwise each and every seat has passed testing with a lap belt only, no top tether. So I wouldn't stress too much about it. You're in the gray area of safer versus safest for that child, and then moreso when you look at your overall configuration. The only thing I might change in yours, if you had a tether anchor in the middle of the middle is to move the rear facing child to the middle of the third row. Otherwise, if there's no tether in that position either, or the MR doesn't install in between the two boosters, then your kids are as safe in your car as they possibly can be, and don't lose sleep over it. If you could get tethers, fine. If you can't, don't stress. Like I said, there are very few seats that fail testing without a tether.
Wendy