Igor987, I'm sorry to hear that you're still having trouble. Do you have another micro USB cable you can try? Also, you might want to make sure it's out of the low-level mode by doing the Power + 20 presses of the Center button, then leave the TP plugged into the wall adapter for an extended period of time (few hours, at least).
Of course, there is the possibility that something is actually fried in your TP, the wall charger, the cable or the micro USB jack on the TP. I think I mentioned the flood-damaged TPs being sold on eBay. I unknowingly purchased one that actually worked when it arrived. I went out of town the next day, and when I returned a month later, the TP was fried, and trapped in a self-test mode. I cracked it open to find the boards and switches covered with corrosion and obvious signs of water damage. It had been submerged in the flood and had an ounce or two of water sloshing around inside. When it was laying flat, the water collected safely under the battery, without touching any circuitry, but unfortunately, it was left on my desk at a slight angle. The water collected in the corner between the Power and Volume Buttons.
All of that was to say that a flood-damaged TP from eBay may work for a short time after the flood.
I know it's frustrating, but you're doing exactly what you should be doing: exhausting all of the possibilities that are within your control. If you get to the point where you do have to call HP, you'll be able to honestly chronicle your attempts to revive it.
Also, don't take the possibility of a bad USB cable too lightly. I've read posts here complaining about a bad HP USB cable, and I've seen bad Apple USB cables that will connect to iTunes just fine, but won't connect to a charger. (On the iPad, I actually get a message saying something to the effect that the connection is not capable/compatible for charging.)
Good luck, and please keep us posted on your progress.