On mounted combat: I'm not sure fighting from horseback is much of an advantage, really. Your reach is shorter (which can be fixed with longer weapons of course) and you have less mobility. Is there any historic sources verifying that fighting from horseback is a boon? I imagine (but could be very wrong) that the primary advantage of mounted combat is in battlefield visability (you're high up and can see what's going on) and psychology (horses are scary).
As for horses being scared by combat - absolutelly! Horses are shitty mounts actually (I'm sure my love for donkeys and camels comes through in the Mounts section

). But I don't think detailing this is appropriate in the rules - as with other things, this depends on the style of play. In a gritty/"realistic" game, frightened mounts matter, while in a heroic one it won't matter.
And finally, one's Ride should affect one's ability to fight from horseback. As a quick rule, I would say that any figthing skills used on horseback are caped by the riders Ride skill (i.e. if Ride is lower, then the effective rank of fighting skills are set to the level of Ride).
On your other comments: Very good points all around. I agree with them all (and good point about the Incarnates

).