Re your problems with the 600 headset...ouch! I hate that feeling when you kind of get half way and then don't know whether to keep going or retrace your steps, as it were. It occurs to me that it seems unlikely on the face of it that it's cross-threaded - would it have travelled as far as it seems to have if it was? Possibly a damaged bit of thread at that point on the steerer tube? Even a bit of grit? Don't know.
I don't know if you can 'retrace', but if you can, and the races aren't too badly pitted, you might consider simply removing and replacing the bearings, which you could do without disassembling the whole headset. It would be a bit fiddly, and you'd have to rip out the bearing-holder-ring (whatever it's called) and just use loose bearings for your replacements, but it would be perfectly sound. Just a thought...
Alan Paterson
London, England