only saw reflection?

When Paul saw her reflection and then ran up the stairs and came face-to-face with her, she was in shadows. Perhaps he only got a glimpse of her and that was enough to give him grey hair.

No- and here's why. That scene followed the spirit of the original Greek legend concerning the method by which Perseus killed Medusa. He was able to decapitate her in the historical legend by looking at her reflection in the polished surface of his shield and guiding his sword home, without being turned to stone. So Richard Pasco, seeing Megaera's reflection in the pool, would not have been Gorgonized, using that logic.

You can't discard a major part of a myth if large numbers of the audience are familiar with the original story, IMHO. :>)