My first instinct is that (if all else really has failed) the next thing to do is disconnect the battery for a few minutes - which will mean undoing the screws which attach the lower half of the case, levering the case open and just undoing the battery connection cable at whichever end is most accessible - you shouldn't need to remove any parts or even the battery itself. Then reconnect after (say) 10 minutes, do up the case and start again with the boot up attempts.
It's a draconian step - but if nothing external in the way of keypresses or button-pushing has worked, it's where I'd go next (assuming the device is well out of warranty).