What will the damage modifiers be for striking while stealthed, both melee and ranged?

Will there be bonuses if an Orcs on it's unicorn, or Breton on it's scary looking cow?

And will there be xtra xtra bonuses if they're mounted and you're stealthed?

There will surely be bonuses for successful stealth attacks, but exactly what the multiplier will be I don't know. Don't expect one shot kills though, unless the target is much lower level than you.

I doubt it will make a difference if your target is mounted or not.