Ok so I took the wheel and cog into the LBS today. John hit it up with a thread file, and it was still catching about two turns in. It was a bit sketchy, but it was threading on dead straight so he just greased the **** out of it and used a chain-whip to get it on. It wasn't cross-threading, but we think the threads on the hub were just a little high for it, so the cog was shearing off the tops.

Kinda scary, but it's on now and hopefully it'll stay on (and hopefully I'll still be able to replace it).

Fingers crossed.