Travel help, please!

In a couple of months I will be traveling with my two sons by plane (and then we will have a rental car), and I am trying to figure out my car seat options. My boys are 4 years/36"/31lbs and 5.5 years/42"/41lbs.

At home, they both travel in Frontier 90s, in the 5-pt harness. For various reasons, I do not want to bring the F90s with us (mainly weight + not airplane compatible). We also have two Cosco Sceneras from travels past, though of course my older son is too big for the Scenera now.

Anyhow, I'd like to figure out my best options for my boys, both for on the plane and once we arrive. I will have family in my final destination who could buy seats for us there, so we wouldn't have to necessarily carry them with us.

Does my older son even need a carseat on the plane? If so, what would be appropriate? Maybe a CARES harness (though I think he is technically too tall)? Or is that silly to consider, since he clearly wouldn't be able to use it again? And once we get there....then what? I read in another thread about the Harmony Folding Booster....we haven't done any booster training, but have plenty of time before the trip, so could. He never falls asleep in the car, so that's not an issue. He is small, though - is 42" tall enough for a booster? Is there another option for a kid who is over 40# but is used to being harnessed?

My younger son could ride in the Scenera, both on the plane and in the rental, but....is there another lightweight, low-cost option that won't make me want to tear my hair out during the installation? Or do I need to just suck it up? (And the FF has to be better than the RF install, right? I mean, it couldn't possibly be worse....)

Any thoughts or suggestions would be appreciated, especially how to keep my older son as safe as possible on the plane. Thanks!

Well at your boys ages and sizes I'd be ok with them just using the plane seatbelt as long as you think they will be ok and comfortable. Why don't you try booster training your oldest before hand to see how he does. As long as you aren't doing much driving he will probably be ok. Pack the high back part of the booster in your suitcase, and have him bring the bottom part on the plane and put it under the seat in front of him.
As for your youngest, I've never installed a scenera so no help there, sorry. If you double check he fits ff in the scenera you could bring that and have him sit on it on the plane, or if you would prefer to have your family buy 2 seats for the boys you could do that. I would recommend the Evenflo SureRide as you can get it on sale right now for around $70 shipped to your destination (from kohls), they have very tall harness slots and go to 65 pounds. Also of you'd prefer a ff only seat to a convertible, look at the Evenflo maestro as I know it has a decent height, harnesses to 50 pounds and is fairly inexpensive (not sure of exact price).

I would take the Scenera for the 4 year old if he fits. The forward facing installation is easier than the rear facing one. You will probably have to peel back the cover to tighten, but that's not a huge deal.

For the 5 year old, I would have a seat waiting at your destination. Harnessed or booster is up to you, you know your child best. If you decide on a belt positioning booster, the Cosco Pronto is inexpensive and tall, but it requires a vehicle head rest behind it. The Graco Turbobooster is another inexpensive option and it does not require vehicle head support. As far as harnessed seats, the Evenflo Maestro is between $60 and $80 and harnesses to 50 lbs.