It seems that it is indeed impossible to put the 8500 back together. The back case is broken, and the clips on the inside of the main case are mostly gone, so the mobo can't fit in correctly nor can the back case go on.
Should I part it out?

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No. Leave it for a few days and then try again.

Use your third and fourth arms to hold the case internals together, and reserve your primary arms for putting the metal lid on. When you screw the case together, everything will become secure. You may need to miss out the front panel reset switch which will break anyway.