OK, I played HC for a looong time and I can tell you everything.
Horse Archery (skill) DOES affect crossbows, too.
There's only one really valid build for dedicated Horse Crossbowman:
Level 30 (4 420 577 xp)
Strength: 7
Agility: 30
Hit points: 42
Unused skill points: 2
Skills to attributes: 4
Ironflesh: 0
Power Strike: 0
Shield: 0
Athletics: 0
Riding: 10
Horse Archery: 5
Power Draw: 0
Power Throw: 0
Weapon Master: 10
One Handed: 1
Two Handed: 1
Polearm: 1
Archery: 1
Crossbow: 189
Throwing: 1
2 free skill points to use for whatever you want, be it IF, athletics or PS.
As to horse... Palfrey if you have money problems and Courser if you have money for upkeep.
Don't even try to use slower horse than Palfrey, because you will end chased by whole enemy cavalry team and won't be able to ride away. With Palfrey (NOT loomed, mind you) I was able to runaway from any cavalry while bombarding their horses with bolts. Courser is even better. Remember, 10 riding skill does increase your manouverability and speed, so such a high riding skill will change your feeling of how to ride a Courser.
The main role of Horse Crossbowman is taking down enemy horses (hopefully riders, too) and harassing enemy infantry as you would with light crossbow on foot - little difference is that you can reload while riding your horse at full speed. As every cavalry, you need to take care to not let your horse die too fast or you are screwed. DON'T bump people if you aren't 100% sure noone will hit you while doing so.
Also I didn't play HC for long time, but I don't think they changed anything about Horse Archery skill. What I am talking about is the fact, that with 5 horse archery skill your reticule at FULL SPEED on courser would be exactly the same as standing still on foot.
I hope I didn't forget about anything.
Regards,
Dravic