Why was coolant added? It should not consume coolant and have very minimal evaporative losses if you have a reserve tank, and none if you don't.
I suppose anything is "possible" but it is very unlikely to have a heater core clog at this young age and mileage with the factory fill, assuming the correct OEM coolant was used to top it off.
Stuck blend door, the servo motor fried or the door itself broke, or perhaps some gasket/etc came loose and blocked its movement, are the most likely.
Driver's and passenger's sides having significantly different temp doesn't make sense without dual zone control, unless it is designed for dual zone as an option so has two blend doors and only one of the two has malfunctioned.
If you are afraid of a high repair cost, why would you take it to a dealer? That pretty much guarantees the highest repair cost possible. Seek a forum where fellow Jetta owners can tell you how much needs done to get to the blend door if it's more than just pulling the glovebox out, or pull the glovebox out and see what there is to see.
If the blend door motor isn't activating, then use a multimeter to see if power is getting to it when you change the temperature setting, to determine if it's the motor or upstream at the switch or ?? If the motor is activating, examine the door mechanism itself.