In reply to Red Rover:
I would have thought it easier just to get straight off the mountain, an extra 4-5miles downhill to aber would be as easy as going back to Foel Grach to stay in the emergency shelter. The paths are pretty easy to follow, even in the dark.
Not sure about buses from Aber, they should run pretty regularly though back to Llandudno Junction or Bangor to catch the train anywhere.
Good luck with the walk anyway, its good fun and there should be plenty of other people about, you may well be able to scab a lift from someone (but obviously don't bank on it!)