The Radian 65 folds and you can get a carry bag, but that sucker is heavy. It works well on the plane and you can use a tray table with it. It is a convertible seat and installs easily with LATCH, which rental cars should have. I think it will fit an overhead bin, so you can bring it on and use it if there is an empty seat. Don't even think about the Radian 80 with those lightweights - no way are they going to get over 65 lbs before they are too tall.
ETA: I think the Radian has a 22 lb lower limit for forward facing, so your younger one may
have to use it RF which most likely won't work on the plane given how tall and reclined that seat is.
The Scenera really is a good option there.
Another option for your older one is a Graco Cargo. It harnesses to 40 lbs and about as tall as the Boulevard. It is much lighter and sits lower to the seat. It is forward facing only, though, so you won't get much extra use out of it at home until she is FF. It is $80-100, just look for one with a front harness adjuster. It is really a nice harnessed seat, but it doesn't fold. You can use it on the plane for her instead of the overhead bin.
There was just an undercover story about rental seats in California, so google that and send it to your DH.
Also explain that for the $8-10/day they will charge you, you can buy two Sceneras to use and not worry about dirty, broken, unsafe seats.