I don't know what is happening to Law, but the way it cut to him right after Luffy collapsed makes me think that both events are related.
As for my guess about the mysterious person..I know it sounds crazy, but I think it's Aokiji. Who else would wear those shoes ?
The Silhouette reminds me of Aokiji's frame as well.
All three admirals wore clothing that matched their powers, those white shoes immediately jumped out to me.