no ideas how and donna know in 100% that its hardware issue (but in must cases it is), because of that we dont know for a fact that what is the product code of yours N9 - this is very important, because i witnes many times when N9 died after some attempts to flash it with uncorrect product code firmware. Also there was such situation, because of that battery was down there was some short inside of mainboard and flash memory corrupted, there are too many ways and reasons. and i'm not persist that its 100% hardware issue,that it cannot be fixed. May be if we know the right product code,then with it can be fixed. There is a nice program Phoenix, with this it can be read\flashed in by dead mode, so it can be finded the product code in dead mode,must try like that example.

Also did you tried to flash with cleaning commands? like erasing