3rd row seat - no tether - who is safe?

I'm still trying to figure out about what vehicle to purchase. One or the cars we like doesn't have latch or top tethers in the 3rd row seats. I can live without the latch, but I'm still wondering about the top tethers. We are looking at a new vehicle so there are shoulder belts and headrests in all positions. The vehicle was on the safety list I saw here for vehicles with the third row seating.

The third row would be for occasional use - mostly if we have family visiting and we want to go places together as a family. Since we keep cars for a long time, children could include newborns to teenagers over the course of the next 10 years. The rest of the time, my twins would be in the middle row with latch and tethers.

So who would be safe in the third row? Who would not be safe? (please tell me why they are not safe.) If anyone wants to rank these by safest to least/not safe, please do...

Infant in an infant seat

Infant rear-facing in a convertible seat

Toddler/preschooler in a rear-facing convertible seat (my DD is 3 1/2 years old and will probably stay rf for another 6 months to a year at her current rate of growth)

Child in a forward-facing 5pt harness car seat (Is there a difference if the child is in a Britax Decathlon/Marathon or Radian? I thought that I read the Radian has lower head excursion so it would be safer.)

Child (6 years or older) in a high back booster.

Child (8 year or older) in a low back booster.

Child who passes the 5 step test for the seat

Adult (who also passes the 5 step test)

Without tethers I would give the ffing harnessed kids priority in the middle row. The rest don't use tethers. I have tethers on the floor of my van. I arranged things so that there were rfing seats behind the tethered seat. That way no one has to trip over the tether.

I'm the OP of this thread and I still have more questions.

So are children in a 5pt harness seat without a top tether unsafe? Or is it just that the top tether makes it safER?

Why is it that the 5pt harness car seat needs a top tether and head excursion is a concern - but a high back booster doesn't have a top tether and head excursion isn't a concern?

One of my friends is in a similar situation. She has her 4 1/2 year old twins in Britax Marathon seats in the 3rd row of the Trailblazer. It doesn't have top tethers (which is the reason she didn't buy two Regents, since it requires it over 50lbs). Her twins are about 42in tall and probably a little over 40lbs. So are her children unsafe in this position? Would it be safer for her children (turn 5 in Jan) to be in high back boosters since they don't use top tethers or would it be safer for her children to be in the 5pt harness without the top tether?

Are some car manufacturers allowing top tethers to be retrofitted with newer vehicles?

Melanie