Re: CARES
Good luck finding any information on Air France and what car seats they accept. They've always told me "no" but I didn't have a Marathon. The Marathon is not sold here in France so I doubt it. No higher harnessed seats are sold, except maybe handicapped seats but I wouldn't know...
Checking car seats in as baggage is a bad idea. Who knows how they treat them and what would you do in Greece if they don't make it on your connection? You not only have a connection but are changing airlines. It's risky...
Gate-checking is supposedly gentler but no guarantee. If they're taken down with the wheelchairs, we would hope so! Logically, it's less likely that the seat will end up being "mis-routed" since it's usually loaded directly and last. I'd take that chance over gate-checking.
Your husband is simply wrong about an accident. More people survive accidents than die in them these days so not having your child pulled from your arms and thown in forward impact is definitely a good idea. The plane is doing about 200km an hour on the runway. Would you hold your child in your lap in a car going that fast? The truth is that children have died in accidents who could have lived if they had been strapped in a car seat and those who actually survived because they were. Statistics? No. First, air travel is so safe that it boils down to individual incidents. The airlines don't share those details because of tact and the possibility of lawsuits. You wouldn't want that info made public if it were your relatives!
When you hold your child on your lap, it's because you're super confident in the safety of air travel and if you look at the numbers, you're right! The chances of anything going wrong are very slim. I flew for a living for 13 years and my heart never quickened (three very minor incidents-not life threatening!). Lost luggage? Now that's more likely
You can wash out your undies in the sink but improvising when you don't have a car seat is another matter...