If I remember Oblivion correctly, when I shot someone who was on a horse, the damage would affect the horse first, before the rider, the horse would die before the rider would, Is this the case in Skyrim as well? Are there any mods to change this?

Also, just a question, but when dawnguard was being previewed, I thought that one feature that was being advertised was fighting from horseback? Does anybody else remember that? I assume that it didnt make it in if it was. Not that I really want fighting from horses, I just can't remember if that was advertised or not, I thought it was but i could be wrong.

Mounted combat was added to the game in patch 1.6 - the patch that preceeded Dawnguard. It was advertised with Dawnguard, but not added by it.
As for the main part of your question ... I have no idea.

Welcome to the forums, by the way!

What was previewed was 'fighting from horseback'...and Beth have in this case been very true to their word. As the only other people I've ever seen on horseback apart from my own huscarls have been a couple of random 'nobles', it's very hard to actually have 'fighting on horseback' with someone else on a horse.

That said, although the term 'khajit trade caravan' has all sorts of romanticised vision to it, it's just four dudes wandering the roads, with magic pockets that can hold tents, chopping blocks, mammoth tusks, weapons, etc like Dr Who's Tardis...would be good to give them a couple of led packhorses too.

I have killed Imperial Noble with one sneak shot and he fell off the horse and the horse wasn't damaged during that.