After several formats/erases/factory resets, I managed to get 7.1.1.0G to install.
When you hold volume up + power, the option to clear data seems to only clean applications' data, which is what should be needed.
However, I had to actually erase the whole internal memory a few times.
That means I lost a lot of data, as the backup doesn't restore itself, at times.
It's a bit of hit and a lot of miss, to be honest.
Not pleased one bit!

it worked for me !!!!! finally i was able to upgrade from 7.0 to 7.1.1  thank you so much for sharing this.... its stable till now and much better than before.

all the best