Thank you all for your help- it was invaluable! Had I gone to anyone else for advice they would have looked at me like I had two heads and said, "it's vacation, let them go without car seats for a week." (Yes, they did say that.)
I "practiced" several scenarios before settling on: Carrying the Radian onboard the plane for the 3yo to ff. Checking two hard-side suitcases to pack the Radian boot and AA, Incognito booster, 2 Connext boosters, and a bunch of surprise treats for our rental car. I figured if those suitcases were lost in-transit I would send dh to get the rental car and go to the nearest big box store for 3 new boosters before picking the rest of us up at the airport. I decided on this rather than carrying on or gate checking the boosters because 1.) With 4 kids and a Radian to wrangle I wanted to keep our carry on to a manageable minimum, 2.) I've had gate-checked baby items broken and it still leaves a bad taste in my mouth, and 3.) when I've had checked luggage get lost airlines have been very helpful compared to other experiences (see #2).
The Radian was a beast to carry, but kept 3yo well-behaved on the plane so was worth it's weight in, well, Radians. (Only other seat I saw while flying was a Nextfit- wished I'd sat closer to them to see how that worked.) The suitcases were on time and intact. Hertz tried to give us a Ford Transconnect, but I balked because it took the rooftop cargo box on our Maxda5 to get us and all our luggage to the airport, so I knew that wouldn't be big enough. The only other thing they had was a Chrysler T&C, so we took that (last time we got a Honda Ody but no luck this time). In the parking lot I unpacked the boosters/treats suitcases (yes, I am crazy, so I'm used to getting those looks) so the family would be happy while I played with seats. I had included some of dh's fave treats, so even he was peaceful. The shoulder belt fit with the Connexts in the 3rd row was horrible, as I'd been warned in this thread, so those went in the 2nd row. The Radian rfing went into the middle seat of the 3rd row. Dd did need the Incognito in the 3rd row, but the shoulder belt fit was ok with that. About par with our 3rd row Mazda5, so at least she was used to it. The only issue we had was the deep slant of the 3rd row. The Radian certainly didn't need the AA- in fact I don't think that would even have fit!, but poor dd was miserable. We had to make frequent stops so she could stretch her legs. And she's 5' and used to being in the 3rd row of a Mazda5 for hours! The frequent stops and surprise toys/treats kept all the booster kids awake, and 7yo sat quite well, which was a huge surprise. And relief.
So it all went off without a hitch and I don't feel safety was compromised at all. Thank you all so, so much!