Troubleshooting my Nexus 5 which went dead for no reason

Problem:
My Nexus 5 encountered sudden death for no apparent reason, it's gone completely unresponsive.

History:

• Using this phone for past 2 years


• I'm a stock user, never rooted my phone or flashed any ROM.


• Did factory reset and fresh install of Marshmallow through OTA last year


• Most recent update to my phone was the July security update through OTA


• My screen shattered in April so I replaced it with an OEM unit (since it was costing me less than half what LG service centre was charging me)


• The connector below the battery was slightly loose, which resulted in only the backlight turning on and nothing else showing on the screen. To fix this, I applied a small foam tape over the connector to ensure it was tightly connected, but not too tight to damage it and the screen has been working fine ever since.

Problem in detail:
My battery was on 14% so I plugged in my stock charger. I launched the Mobdro app to watch a live stream for a good 10 minutes. Then suddenly the screen turned off and the sound stopped at the same time.

Troubleshooting:
I did the following but nothing works:

• Faulty display ruled out - I pressed volume buttons all up/all down to sense vibration mode, double pressed on power button (in the hope that I will get a vibration when it launches camera) but I don't get any sound or vibration.


• Long press Power button to restart phone, no response.


• Power + Volume Down for 30 secs, no response.


• Power + Volume Up + Volume down for 30 secs, no response.


• Changed USB Cable, Charger to ensure battery was charging and left it on charging for 60 minutes, retried all above steps, still no response.


• I checked my charger and cable (which were being used when phone went dead) to charge a different phone and they are working fine. I'm guessing that this can't be a voltage fluctuation problem otherwise my charger would've been damaged and would have stopped working.


• Connected through USB cable to my PC, I get a "USB Connected sound" and then "USB disconnected sound". This sound loop keeps repeating until I press Power and Volume Down for 10-15 seconds, after which I get a USB disconnected sound and USB Connected sound and then it stops. Then I get a message on PC that installation of drivers failed for unidentified USB device.


• Dead battery ruled out - I took my phone to a hardware store, the first thing he did was to disconnect battery and charge it externally. Then he tested the Micro-USB port, motherboard, internal charging cable and those were fine. Which means that battery was in fact being charged through the phone.


• Took it to an LG service centre today where the person refused to inspect it because the display is not original. I reasoned with him to inspect and if they find that the display is the problem they can change it and I will pay, he asked me to come back later as his owner (who presumably takes these decisions) wasn't available.

I'm unable to find out the root cause of this problem and how I can fix it, and I'm scared that LG service centre will charge me huge amount even if it's a simple fix. Please help me in reviving my beloved Nexus 5 to work again. Is there something that can be done?