It's a moot point really after all those posts. That being said, bringing a car seat and having the kids stay in there is a scenario that works at a certain age. E.g. DS recently did great in his Radian during a transatlantic flight (3.5 years). DD, a lap baby, and yes, selfish me wasn't able to afford another $1400 ticket, did good in my arms, but would be furious for a full 10 hours if restrained in a car seat right next to me. SUre safe, but in my experience, kids between I just started walking and 24 months don't understand the need for restraint. No bribes will work when there is no level of reasoning there just yet. My SIL traveled just from SFO to Hawaii with her daughters around that age and they kicked and screamed the entire flight. I put my travel dates so that we won't fly again to Europe until DD is around 2.5 years old - she'll do great in her car seat then.
Also, it is impossible to not get out of your seat during a 10-13 hour flight. You will have to pee eventually and the kids need to pee too or you need to change a diaper. But a turbulence could happen anytime. So in that logic - stay in your seat with your seatbelt buckled and pee your pants. Better safe than sorry ;) --- I understand the risks. Flying statistically is much safer than riding a car. And evil selfish mom I am - I take them to preschool/to the market/ lessons every day in their rf-ing seats and statistically, that's a very bad thing.
Last but not least, OP, I have checked car seats. I didn't know about the risks at first. I never had issues and examined my seats closely. During my last flight, I brought a Scenera and left it at my parents' house for the future (cause I also have CARES harnesses). I checked it. Lufthansa handcarried it to the plane and back to the baggage claim. They were very cautious with it. A Scenera can easily be looked over for cracks due to its simplicity. But I will never again be faced with this dilemma - DD will be 2 soon. THough I seriously HATE lugging car seats around the airport. But I have learned now to ask for help - airlines provide it and let you store the seats at the gate prior to boarding so you can roam around with your kidlets. Lufthansa didn't even "allow" me to carry, the guys insisted they will carry the thing through immigration/customs for me and safely put it down on a luggage cart!