Problem solved, though I don't know why or how.
Using Revo uninstaller, I scrubbed the drive and registry of everything to do with ASCOM, EQMOD, PHD2, and SGP.
I reinstalled everything in that order. The mount is now tracking, with SGP working well with PHD2. Metaguide, on the other hand, still has an issue of a lost EQASCOM connection as soon as a sequence in SGP is executed.
Thanks to all for your time. The mystery remains as to what may have been corrupted, but it works.