Heater core problems/ not blowing hot air from passenger side

Hello, Scotty. I've got a problem on my VW CC 2.0T (2012 facelift) model. Backstory to this issue is that my coolant was missing every 40 or something miles. I went to see my mechanic , to see whats up. He then decided it was due to radiator leak or something, so he put a little bit of radiator sealant in my coolant tank. It did not help. Then when i came back to him, he decided to put some other kind of sealant ( it was like little flakes of something inside my coolant tank for a awhile). Then it happened, ( my coolant was still missing and my car did not heat anymore(its cold now here)). So i found a guy who fixes all heating issues(or related to it) at his garage. So i went to him, he found the REAL problem, turns out my water pump was broken( and flushed the system a few times ( because i told him what other mechanic did with the sealant stuff to my car)) and my missing coolant problems went away. But then another problem came up.. the heat wont come up from the passenger side, it is like lukewarm(coldish) and on the drivers side it blows hot air as it should be. And like half of my windshield is always fogged up ( on the passenger side) . Now my new guy said it is maybe a heater core or either a blend door problem. What do you tink? It is possible to flush the heater core on this particular model(VW CC)? Thanks for your reply. Big fan.