Heater cores usually do not just "crap out". They slowly clog or start to leak. If you are suddenly getting no heat, I would suspect a different problem. Is the blower working? That is a far more common problem. Been there, done that.
Yup you'll have to R & R the actuator. I have never had to do one but it is kinda tight in there as you have already seen. Search for a post about actuators. I think Ewill3rd once talked about it. No idea on price.
I have never had a clogged heater core yet, though I have read about some. I would think flushing is the best medicine.