So after restoring to 5.1.1 and fiddling around with QFIL and constructing things from scratch myself the solution is as follows and most likely applies to any variant of these devices.
1. Hold the volume buttons and PWR buttons until the device vibrates. The screen will be powered on but black instead of off which is in EDL mode. adb reboot edl does not work. I'm not exactly sure how the OP managed to use that unless he had an older build with the functionality or perhaps ADB removed the ability to pass the argument in newer builds past X date and I currently use the latest. Either way this is the only method to get EDL mode without opening the device to my knowledge as EDL cables even did not work for myself.
2. You need to flash within roughly 10 seconds of entering EDL mode. The window is small and you will see a 3 second timer in QFIL if you got it right. If not just reset the device again with step 1. You can have the device plugged in prior to entering EDL mode and QFIL pointing to the proper directory in advance to make things easy.
3. You need to modify the rawprogrammer0.xml that is provided as its making references to files that do not exist. File types that do not exist such as sparse images of which system.img is not a sparse file. I made the mistake here initially by commenting out lines instead of just removing the referenced file which soft bricked the device. I've uploaded the modified file which should work fine for any device and also retain user files. I would the original worked for the OP due to its his cached files/content/userdata but it most definitely not work for others.
Appreciate the firehose file as that was the missing link to this and other same model files would not authenticate for this device despite being the same chipset. I've restored functionality with the provided xml and flashed a secondary tablet without any issues and should be fine for others as well.