just a dumb idea here, but have you tried to flash 3.55 OFW directly? If i'm not mistaken, TechnoDon said that jfw shows 3.56 because it's SPOOFED to show like that (perhaps they themselves added patches in order to prevent downgrade to a version != 3.55). Try a normal 3.55 update from the usb disk. If it hangs at some % , check the error in the wiki and report it here.
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