Just to report back after some months on anyone that might find the thread helpful.
1st step: Coating removed completely - no help, wipers were skipping like crazy again.
That lead to only one reason - faulty design. A lot of threads in MB world forum with people complaining of skipping Benz wipers with or without coating. Also another thing - driver wiper was hitting almost each stroke the A pillar.
2nd step: Go to Benz dealership, complain on the driver wiper hitting A pillar and wiper skipping in light rain now that coating was removed. They did blame the coating at first , of course.
Solution: They fixed the wiper hitting A pillar and reported that they did not do anything else like adjusting wiper arm.
Afterwards: Skipping just almost entirely stopped ... yes I do hear it once in a while with less rain drops and it's definitely not gliding perfectly , but as I said it seems like this is a common issue with GLCs wipers and I have to accept that little flaw.
So it seems like the issue with skipping was only with the driver wiper and I guess once they fixed the A pillar hitting they did fix the wiper arm itself, although they didn't report that. Maybe fixing the A pillar did a fix on the arm as well, who knows.
I feel a bit stupid I didn't go to the dealership earlier as the coating might have not been the issue at all and I might have kept it, but to be honest - I would avoid putting anything on a windshield again. Too many unknown how the glass might react.
Unfortunately with the coating removed now light windshield scratches are already visible, but not much that can be done.
I know I said I wouldn't put a ceramic coating on a windshield again, but just curious - what do you guys think? Do you think I should reapply now that the wipers are fixed or I might fall into the same trap? I did enjoy the water repelling effect, but definitely don't want to go through the same process. Also it has the added layer of protection against wipers scratches.