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Summary
Gone through this again and [Only registered and activated users can see links Click here to register] did not work for me this time. Tried flashing 2.6 but I could not get past "patching system image unconditionally".

Situation

• Stuck in bootloop
  

What failed to work

• adb sideload (stops midway)
  
• Wipe /system & /cache (-> Unable to mount /system & /cache errors in TWRP)
  
• Directly using fastboot erase/flash
  
• Countless driver installs
  

Working Steps

1. Connect your device to your PC.
   
2. During its rebooting cycle hold power + all volume buttons (up-center-down). Keep them pressed till it finally stops rebooting.
   
3. If device is not present in Device Manager under "Ports (COM & LPT)" then disconnect and restart from 1, else proceed. You may want to check "View > Show Hidden Devices".
   
4. If "Qualcomm HS-USB QDLoader 9008" or similar is present under "Ports (COM & LPT)" but not SHOWING AN EXCLAMATION MARK on it then perform additional steps 4.1 and 4.2, else skip to 5. Do not worry if exclamation mark disappears after a while.
   
   1. While it's connected uninstall all "HS" or "Qualcomm" drivers from Device Manager (check "Delete the driver software for this device"), even for those that get automatically installed.
      
   2. If drivers were uninstalled then disconnect and reconnect device to PC. Restart from step 2.
      
   
5. Place the whole "emmc" folder in root C:\ drive. File paths with spaces are not supported by QFIL and will also throw a Sahara error so be careful.
   
6. Set up QFIL [Flat Build > Programmer Path > prog_emmc_firehose_8936.mbn], make sure COM Port is recognised by QFIL and click "Download". Wait.
   
7. If download fails with "Download Fail:Sahara Fail:QSaharaServer Failrocess fail" then re-click "Download" and immediately hold power + all volume buttons. Release pressed buttons when "Switch to FireHose. Wait for 3 seconds..." log shows to start ROM transfer. You may have to repeat this step multiple times.
   

I'm probably recycling what's already been posted. Didn't really re-read through all the posts.