I made it all the way through Mzulft and the Mzulft Boilery to the Mzulft Aerodrome. There, I inserted my focusing crystal into the Oculary, but then left the room, cleared Mzulft of all metal to smelt, and returned later. When I returned, I couldn't make the crystal expand or contract using any sort of hot or cold spell. The crystal was unaffected by my spells.

I have seen a few post's detailing the same problem (although on different quest's) in which they left at a certain point only to find they could not continue for some strange reason when they eventually returned. The spell book's that you see on the table just behind the 3 switch pedestal's, i assume you have tried them?

Put the Fire spell in the Left Hand and the Frost Spell in the Right.
Then Cast both spells at the same time on the Crystal.