Humans can end up chain of commanding into a squad with a siege engine.
Thus a cannon can end up crewed by people with different Ranged attack scores/abilities. what determines the ranged of the shot, 1 chosen model? the majority of the crew? or the most unskilled crew-member?
Mark is correct on who shoots the weapon. Interesting idea with the siege weapons and mounts. Be curious to how it plays out.
As far as I am aware, you may choose a model in the squad that attacks with the siege weapon in question. As long as the class joining a lower squad with chain of command had acced to the weapon in question via a trait or ability, they are able to crew it with the lower level models. For combat enhancements, 50% of the squad needs to qualify for an enhancement for the squad to get it (i.e. a squad of three Strength 2 guys and one Strength 4 guy wouldn't get the +1 bonus to their combat situation)
As for the mount question, I don't see why not, as long as the whole crew is mounted, although you'd still only be able to drag it at your base movement, so at most you'd be getting an extra inch or 2 out of it. If you want more mobile artillery you're going to have to invest in wagons
Incidentally, if one has access to horses & cannons, would it be possible to relocate the cannon by horse limber? obviously while still subject to all normal siege weapon restrictions? by equipping the squad with mounts and siege weapons?