can I use an unused y2 or w2 aux wire and connect to the C terminal
It looks to me that you can connect either wire to the same terminal as the yellow wire. I would use the brown simply because it is as close to black than any other color. To be honest T'stat control wiring really doesn't have a color code but everyone sticks to the color that the letter indicates for clarity's sake. So disconnect the brown from your control panel and connect it to the C terminal and then connect it to the C terminal in the T'stat.

Good luck