Yes, I'm also confused on what is very uncomfortable and unsafe for someone that size. I am 4'11 woman (100lbs now but was in the 90lbs before kids 😆 ) and just sit on the plane's seat like normal people 😬.

I have small kids also and my 12 yo son is even smaller than your 12 yo daughter at 4'6 and 75lbs. He still rides in a backless booster in our car, but he would have died of embarassment if he has to sit on the Britax Frontier in the harness mode on a plane. And at 4'11, isn't that already over the max height of the Frontier harness mode? My 7 yo uses the Frontier in our car in the harness mode and he won't be in his car seat on the plane. We have 20hr+ flights in a week and they both will just sit normal on the plane seat with the normal seatbelt.
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When the kids were younger, we used the Cares harness when not bringing a car seat. But it's only for kids up to 40inches and 44lbs. I just checked their website now and they have one for special needs for kids larger than 40' and 44lbs, but it looks like you need FAA exemption letter to use it.
​​​​​https://www.amsafe.com/product/cares...eds-travelers/