Hi,
I see car seats coming up as checked baggage all the time, so i think that will work ok. I don't think that the airline charges for a car seat, but I would check first.
I would check with my ped about tips for travelling/kids. Mine gave me good tips for the first time I travelled with my newborn.
Now it is time for thinking out loud. Maybe my brainstorming will trigger an idea for you!
And about the sleeping . . . I have seen kinda like a base with thick bumpers that would contain a newborn. The sides were high and thick enough to work. They are bulky though
This might sound silly, but even a small inflatable wading pool might work, it could be packed in your luggage and inflated after arrival. It might have an odor when it is new.
Gee, maybe I should just invent a kid-safe blow up crib!
Anyway, there are laundy baskets, which is what I was placed in as a kid. Might be most feasible to check with the house/condo owner to see if it is furnished with a crib or if they could provide one.
Maybe your brother could rent, borrow, or buy one from someone in PR. I know I would be scrubbing it down though.
Sounds like an adventure. Just take care of yourself. The advance planning is good. Most of all, have fun!