Anyone know if these are hard coded in? Would there be any way of changing it that you would always perform a 'slit throat' type finishing move on an opponent if they're completely unaware you're there and you have the appropriate perk?

seems crazy that it becomes almost random that sometimes you perform it and other times you just whack them really hard and they die.. :/

If you're sneaking and have a dagger strong enough to 1-hit them with the assassination critical then you can already guarantee the throat-slit animation. Just be directly behind them, as close to 90 degrees as you can get, be within short melee range of them, then begin to move towards them while holding the attack button.

If you were close enough and at the right angle it will always perform the neck-slit assassination (although whether it's First Person or Third Person seems random).

Of course the downside to this method is if you are at the wrong angle or too far away you'll end up doing a forwards power attack, which will cause you to awkwardly run into their backside before swinging, which blows your stealth and doesn't give you a sneak attack critical.

Yes this is possible, there's someone on the Facepunch forums who made a mod that replaces the slit-throat killmove with a different one, and there is another mod out there that makes it so you only perform first-person kill moves.

I'm not sure about specifically causing a specific kill move to happen all the time though outside of how it occurs in game, save for disabling all the kill moves except that one.