It seems that the battle is over, which guy will be the first to present a flashable, stock based Firmware
I guess this device has become "too old" and the Nexus 7 ( 2013 ) forums are not that crowded anymore
Download the Factory Image, unpack it up to the point where you can see eg. boot.img, system.img
Flash the bootloader first through fastboot, continue with the boot.img , system.img after.
Make sure to have the latest TWRP recovery.img for your device too since some people lost their custom recovery by flashing system.img... for whatever reasons.
Last but not least, restart the device in the TWRP recovery, flash supersu and finally restart the device
This method requires some initiative but the results are the same : you have "dirty flashed" Android 5.0.2 ( Stock ) --> Android 5.1 ( Stock ) without any trouble