Hi, guys!
So Apple finished investigation on my case. In total, I had 4 calls with them. The first call was very positive and gave some hope that this problem might be solved very soon but unfortunately, the last two ones were not that positive. Customer support sent my reports to the engineering team and they told that it's a hardware issue because everything works normally. I tried to explain how it's possible that after three months, my camera started working for a moment when I added camera apps under "Security & Privacy" --> Accessibility (one user suggested this solution earlier this year). Customer support asked engineers to dig into this question even more but today they called me back and told me that they think it's still a hardware issue (not giving any other specific details about why) and I should let my laptop to be checked at local Apple Care. However, I asked them whether they have had any experience with cases where next major update (I mean Sierra, Mojave, Catalina) may fix the problem and they told me that there have been several cases where next major update solved the issue.
I still doubt that it's a hardware issue because how lagging and not properly working Facetime (left panel doesn't open in normal mode) is related to camera issue. Even if I open Quicktime player and try to record a new movie, I can start recording but it freezes and it doesn't count seconds. Of course, I understand that every one of us has different things that are a bit different but it feels like the core issue is still the same. After certain updates (was it 15.2, 15.3 or 15.4) camera just stopped working.
So my next steps probably would be going to local Apple Care to check the issue and test the external camera. If they find that all cables are working properly then I won't allow changing my display/camera. If next major update still doesn't fix the issue, probably will change it then or buy a new device (not sure if it's going to Apple anymore). Before my Pro, I used Dell Vostro for 7 years. It dropped to the ground at least 5 times during this time and everything worked well until the end despite the major external damages. Now some guys in Apple say that my Pro may have hardware issue although in two years it never dropped down or got any other damages (dust, liquids etc.)
All in all, I still have hope that someday this problem is going to be solved. The last update showed that Apple has some problems with cameras. I will keep posting to this thread if I find anything useful or if indeed it was a hardware issue. Hope you are guys doing the same. If anybody finds anything, I appreciate a lot if you share your experience. We are on the same boat. Good luck!