As it works now finishing moves can only happen on the last enemy of a group, while npcs/companions can get finishing moves randomly nomather how many people are still alive. Is there a way to make the player behave the same way?

The only thing I've found is to increase the chance of finishing moves from the original 50% to 100%, "set killmoverandom to 100", but that still requires the condition that it's a 1 vs 1 fight. So is there any way to remove this condition?

I don't see why you would want to. While you are doing the finishing move, things can attack you. That's why they have it so that it's the last combatant in the group.

As far as I know you can't attack someone else in the midst of a finishing move. But since most finishing moves are only 1 or 2 seconds long I don't see it being much of a problem either way. But the question wasn't what do you prefer, but can it be done.