I know this thread is ancient, but sometimes I find old threads where people are having the same problem I am and I always wonder what they did in the end so I thought I'd come back and update.
In the UK: We lucked out and found a UK-based friend with an outgrown ComfortSport that they
gave us to use for our trip and the flight back. This was awesome as it solved our problem of not having an FAA-approved seat for the flight. As a bonus my son fit great in this seat, I got excellent help here on learning how to install it, and I ended up liking it as a travel seat a lot more than I thought I would from reading posts.
We decided to buy a dedicated car seat in each country for each set of grandparents.
In the US: We bought a TFP. I think this is the ultimate grandparent car seat and the right US seat for DS, but I am still not sure it was worth the $$$ for what is right now a very fancy occasional use seat. I guess I'll have to wait 6+ years to decide, seeing how much use it gets etc. I really love the TFP now. The only major drawback to it for us so far is too wide for 2 adults + the car seat in the backseat.
In the EU: We picked out a Britax Hi-Way for MIL. I really thought I would buy the Two-Way since DS is tall, but in the end I wanted the Hi-Way for the entirely pragmatic reason that I hope to convince SIL to use it someday for her own LOs and didn't want her to have option to FF it at 20 lbs.
I struggled with the install of this seat, but with Adventure Dad's help I got an install I feel is acceptable and MIL decided to leave it installed in her car so I'm glad we shelled out for a legal seat for her!
I also bought an Avenue on sale to swap out for the ComfortSport when DS outgrows the short shell, but I am not thrilled with that purchase. I think we may go straight from the ComfortSport to the Radian for a travel seat. We'll see.
Some of the reasons I picked these particular seats ended up not being relevant and made me wonder if I made the right choices, but I'm happy with our car seat situation now. And I'm glad I never bought a Radian because now I have the option of getting an XTSL with that beautiful flora cover when DS outgrows...something.
Thanks for all the excellent advice on this thread. As you can see, I decided not to ship or check any car seats after all and I feel happy with out decision to use legal seats in each country that are either permanently installed (MIL) or that the owners/users know how to install themselves (my parents).