It will pretty difficult to tell you if the heater core was replaced after being in there for 2 years.
I would find a shop and have the heater core flushed before spending all that money. I have done hundreds of heater core flushing with success, I use a garden hose and some air pressure, go back n forth through the tubes.(also add small piece of heater hoses into an empty bottle, so I can see what is coming out). always clear up, (just limit the air pressure so you don't blow the heater core apart).
The heater core can be partially plugged, that is why you have heat on 1 side but not the other, just because you had this problem before,doesn't mean that it is the same problem. blend air doors and actuators do go bad. listen carefully, when you switch from cold to heat, you will hear an airflow change.if not the door isn't moving.
FLUSH it first, before spending the money.
Go to an independent shop and have it done.dealer is not the best place to go for a flush.

If you are plugging up heater cores this fast, you must have some nasty coolant, you have way more problems coming.