Right, from my experience and from what I've heard from others, the bare minimum kids have actually had most lap belts tighten around them is 35 pounds. And that was one kid, I think (trying to remember whose). Piper was 38-43 pounds before the belt fit her on planes with consistency. So my personal experience and what others have said, along with the FAA recommendations, lead me to recommend a carseat on board under 40 pounds.
Piper also then took three years to figure out how to sleep on the plane. So for comfort's sake, I'd take the seat as well.
Given that the CARES doesn't tighten the lap belt, I'd be uncomfortable recommending it for a 34 pound child, since the whole problem isn't that she can't sit still (at her age it'd probably be ok), but more that the lap belt won't tighten all the way. The CARES harness does nothing to tighten the lap belt. Nor does it have a crotch strap.
I'd take a Maestro and whatever wheels for it you have/want, then have her ride in it. Laine *could* walk through the airport. She'd stop at every shop to look around, would just stop in the middle, would go and talk to people, anything. She generally rides in her seat in the airport (maybe or maybe not strapped in) and it keeps us moving. It's her choice, too. She prefers to ride in her seat, or on my suitcase.
As for the GGB, I don't know if it counts as a carry on. When I bring my folding cart (haven't in years, will again next week) I've both gate checked it and carried it on. No one has mentioned it counting in the least. I would check that you'll get the gate checked items back planeside. For some reason when we fly home to Seattle all gate checked baggage gets sent to the carousel in Seattle. We don't get it back. On other flights we do. So ask the attendants first where the gate checked luggage will be returned, then either carry it on or gate check it. I'll be doing the same next week.
Wendy