You are right, th Aer is IATA approved and should work as a cabin luggage anywhere as well. It is a very good stroller for the type, one of the best, I would say. I honestly don't like the Yoyo much - because of the short backrest that is SO slanted, the baby always halfsits or half-lies, which is dangerous for babies who don't sit yet, and uncomfortable for those who do. The seat is rather small and overall, for the money, I feel like the Aer+ is simply the better, more long-lasting choice, practicality-wise. So definitely recommending that one.
About the carrycot accessory, it depends on the use case and the baby's age. If you have a total newborn and plan to use the stroller a lot, yes, a carrycot is a good thing. With the Yoyo, a must, as it doesn't lie flat enough. With the Aer+, it's more about the comfort and safety, as the seat reclines more so is usable with a smaller baby with a cocoon or goof footmuff as well. If you'd be strolling with it every day and a super small baby, yeah, I would take the carrycot. If the baby is ~4 months and you'll use the Aer in the reclined position, then I wouldn't.