There seems to have been a misunderstanding here somewhere, though for the life of me I can't follow what it was, exactly, nor does it matter that I can't.
However, I can tell you that AA changed their preboard policy several years ago, and their current policy is that they no longer pre-board families with young children on *any* route, carseat or no carseat. The business of whacking another passenger with the seat on the way by tends to be a non-issue for AA on a policy basis, but I find that if I tell a gate agent ahead of time that have difficulty avoiding hitting people with it, there is a fairly good chance that an AA employee will put the seat on the plane for me before anyone boards (usually the ramp agent, that person you see going in and out of the jetway door between flights.)
SWA normally preboards families with young children even if they don't have a carseat to install. The MCO routes are sometimes the exception to that general policy, because there usually are a LOT of young children on those flights. However, regardless of the route, if you need to install a carseat, SWA *will* let at least one person preboard to install it.