E.C.
can't answer your q about fabric and sweating, but the bent-legs-while-rear-facing issue is generally not a problem for little ones - they're much more flexible than we grown ups are, and it's actually not a major safety risk either. My now-9YO twins rear-faced until they were almost 3YO (would have rear-faced longer but their seats had a 30 lb. rear-facing weight limit). As long as your son is within your seat's rear-facing weight limit and has at least 1" of hard plastic shell above his head, he can still use the seat rear-facing (see explanation here: http://www.car-seat.org/showthread.php?t=30201)
a few helpful references:
http://carseatblog.com/5168/why-rear-facing-is-better-you...
http://www.car-safety.org/rearface.html