Yeah... the whole mane-being-weird=bad-hind-end-work makes very little sense. Some of the time it's just because your horse has a really super thick mane (though that seems unlikely in the case of your TB). And what if all of the mane lies perfectly on the left side? What would that mean?
This leads me to a question I've had; is it more desireable to have the horse's mane lie to the right? Why is that?