Sorry, I only read the first 6 pages, but here is my advice:

I have an N810, the board has a "2006" label printed on it, it has 2.2007.50-2 firmware. None of the "G-spot" and charger/battery connect/disconnect tips worked for me. Even letting the IT down for a while. Here's my method, which worked for me (only this time, once; I can tell you, I'll never switch this thing off again):

Put the tablet into the fridge for 15 minutes. At 0 - -5 Celsius I could turn it on.

Next time I'll sacrifice a chicken at full moon or something, I think that will help.

(PS: Damn Nokia, how could it let these devices out of factory??)