You have an idea there. A booster for the oldest child could easily be purchased.
No, Avis and Hertz can be just as bad as any of the others. Too many horror stories about car seats missing parts, dirty, expired, etc. There is probably no regulations for this in Morocco.
You might want to look into getting a Rider Safe Travel Vest for the younger two. These are small and easy to pack. They do get the nod from Car Seat Techs. I'm not sure which countries they are sold in but you could then use them at home for car pooling or babysitters after your trip. There's no side protection but they'll work in any car with a tether OR shoulder belt.
http://www.safetrafficsystem.com/
The Cosco Scenera is a good seat, if you live in N. America. You can use it on board or gate-check it in a bag (don't check it as luggage). At $40-50 U.S. it's probably covers a few days' rental costs, without the risk. Very light and easy to carry. With two parents, taking one car seat shouldn't be too much of a problem.
You may want to go over to Car Seat.org. They have a couple of forums where you can ask this.