It certainly is. I believe it's only limited by the amount of space in the UI (which, obviously, is not a lot).
I was pretty sure that was the case. I've seen a community map that featured the troops training ability available to all players on the map, but that was made possible by scar coding unique to that map. And the way how the king tiger was switched to a dispatch ability that was added to what would be the 6th ability position, I presume.
Sure, why not? Just put them all in army_bag/abilities and give them appropriate requirements. I use required_all_in_list containing several required_player_upgrade that only one commander has in that combination.
So I'd clone a 6th ability and in the requirements add the upgrade requirements of all the other abilities of that specific commander. Also, the 6th ability should be one that has the highest command point requirement, since it would require the activation of all previous abilities, yes?