Here is hopefully the last update on my case.
After few more useless calls back and forth with TPG which were going nowhere I started getting an impression TPG is waiting for me to give up. 
Eventually I was told they are waiting for approval for technician re-visit.
On Tuesday, March 10th (one week after the the first visit) I was told revisit was approved and it will be scheduled soon.
Few hours later I got a call and I was told the technician is in my area today and they can send him there in an hour if I am available. Lucky I was available.
And what a difference!
The second technician went above and beyond my expectations.
Seeing my interest in the topic he started explaining me what and how he was checking.
He tried to follow the line and found another abandoned socket in the bedroom pushed into the wall and hidden behind blank faceplate. The socket was still connected to the line. It was heavily corroded and probably the number of recents storms became the last drop for this poor socket.
The socket was removed and wires were clamped and sealed.
He said he rechecked all connections in the MDF room and even cleaned up disconnected pieces of wires which were still hanging there since my switch from ADSL.
The modem was restarted, connection was tested by backend team and everything looked promising while he was writing the report. He asked me to recheck it before leaving and felt he really does care about things.
And this is the happy end.
The modem is running for 55 hours straight by now. No dropouts. I got 54 upstream error packets and 6 downstream error packets ONLY since the modem restart which is nothing compare to tens of thousands of errors I was getting before.
Everything is great again.
I want to thank that TPG technician again and whoever send him here.
I actually asked his permission to put his name here but I am still uncomfortable posing someones personal information online.
Honestly, everything would be so much easier and faster and more efficient if this technician would be sent here first time.
This would save everyone a lot of phone calls, a replacement modem and some nerves for myself.
Thank you for your time.