I initially posted a longer response but it didn't post here so trying again.
Your cars symptoms. Runner Flap failure. It could be the motor or so much carbon or gunk in the manifold that it has remained closed. Hence why you can;t make boost and running rich.
If you are on 2+, going down to 2 will make absolutely no difference.
To fix the problem, the mechanic will need to remove your intake manifold, diagnose the runner flap failure, probably have to clean the intake ports.
Failure could be too much gunk and carbon on the flaps or the actual runner flap motor is dead, also check that the runner flap motor is actually plugged in. It is labour intensive so it will be several hundreds of dollars but I don;t know what it would cost in South Korea.
If you bought the car with an APR tune. Contact APR and have the Vin number transferred to you, it will cost maybe $150 USD, but you are then entitled to down grade or switch tunes of the same stage or lower for free and it will only cost a mechanics time. (I have done that before with my APR tuner)
If you are up for it, get a Runner Flap Delete Kit (RFD) and get the stage 2+ tune with RFD. That way you won't have this issue of bad cold starts and misfiring.
Contact APR to adivise what file you have, the file that is for Stage 2+, High pressure fuel pump, dump pipe and runner flap delete is this
VW Golf R EURO MY2015 2.0T FSI CDLC 1K8907115J S0020 Stage 2+ FP V3.4 Testpipes [RFD]
FP = Upgraded High Pressure Fuel Pump
Testpipes = Dump/down pipe installed.
RFD = Runner Flaps have been physically removed.
Also downgrading to Stage 2 means you are not running the upgraded fuel pump nothing to do with the RFD.
but you can get a stage 2 file that has RFD tuned in.
Hope this helps.