Congrats on the cats reaching such a ripe old age! You don't state it outright, but your questions seem to imply you expect to fly. But, given you also talk about Latin America, have you thought about driving? As in, if you found a cheap, mid-size RV you would solve your housing issue and could make taking the cats with you an option. (or course, with lots of research on importation / quarantine / vet care in your destination) I just moved to Ireland from the US for an assignment, and we brought our 10 & 13 year old cats with us. While nerve-wracking in terms of handling, they took to it quite well.
I'm not really sure about your question #4. Are you asking if car seats "work"? Or about how to take them with you? We have our US car seat with us, and have found our LATCH system seat fully ISOBAR (European standard) compatible. The only extra is that ISOBAR allows for the 3rd anchor to be a sort of kickstand, rather than a 3rd anchor point. But any newer car, likely designed on a global platform, has the third anchor point in the rear seats.
On a plane, we brought the infant seat with us. The 3 year old could also use a car seat, but we chose a harness instead, both to be more nimble while transferring in the airport, as well as limiting his ability to kick the seat in front of him.