Also, on a side note, I believe British Lady was using the collective You while speaking to Sam. I think she was there to bring both brothers in. She did ask about Dean's whereabouts, after all.

I've seen that interpretation elsewhere. Whether or not she did intend the collective you, the actual speech is given to only Sam. After seasons of only Sam being blamed for the Apocalypse and "betraying" Dean and releasing the Darkness, this is simply another instance where the blame is stated only to Sam. It is also done in an episode where Cas is totally absolved of doing anything wrong by releasing Lucifer, in fact he is praised for it. If the show wanted to clarify that she was blaming both Sam and Dean, it was easy. "We have been watching you Winchesters for years". But it was left deliberately without any other descriptor that would state it was both Winchesters, not just Sam. As for asking where Dean was, if they have been watching for years, they know that Dean will do anything to protect Sam and the question could just as easily have been asked to determine if she had to fight Dean to get Sam out of the bunker.