You appear to have spread your problem over at least three threads, so it’s hard for anyone to keep track of what you have, or have not done. And you haven’t really said much about what you’ve tried. So it’s understandable that people will start with the basic debugging steps first.
If all else fails, you could open the case up, pull out the hard drive, and connect it to a PC to retrieve the files. From your description, it sounds very much like your data is still there, only the OS has become corrupt during the upgrade.
Recovery can be done fairly easily:
Having recovered the data, search for the ‘debricking’ or ‘unbricking’ guides that can be found on the forum, and re-create the OS disk.
