Atwell is back in London, based on her Twitter. She spent two whole months in LA (from March 16 to just recently) working on this film. Even a One Shot should not have taken more than a few days of that time. I still hold out that her role is more substantial than most people think.
VanCamp has been very quiet about her schedule, but based on the tabs, she hasn't really left LA yet to go shoot anywhere. If you go back to the Deadline article that announced her casting, they DO call her the female LEAD of the film, but they do NOT call her the love interest. All of the press reports stem from this one article. The only other fully confirmable thing we know about her part is that she will play Agent 13 (that's from the Marvel press release), but NO WHERE reputable has it been confirmed that she is playing a character named Sharon Carter.
We also know ScarJo has a very big part in the film, and in fact many believe she is the defacto female lead.
That's it. That's all we have to go by. Ninety five percent of everything else is speculation on our parts. You have to go back to the original announcement on Deadline--everybody else just took that very small bit of information and piled on and on.