Re: Agent 13
I think that a large part of the decision to use Natasha instead of Sharon comes from people's desire to see a female superhero on screen. After the Avengers, Natasha is considered a superhero in a way Sharon or Maria or Coulson are not. When media outlets ask Marvel when they are going to release a female-led superhero movie, Marvel can point to Black Widow being the 'co-lead' of Cap2 (ridiculous, IMO, as the trailers seem to show that this is clearly Steve's movie).
I'm pretty sure that Sharon could have been used in this story instead of Natasha without any huge changes - the two of them have a lot of similarities. But using Natasha is better PR.