What are evaluation criteria for NSF postdoctoral fellowships?
I think this depends to a large degree on how intelligently you can write about this subject.  If you have a carefully thought out research plan, and can point to how your previous experience prepared you for it, I don't think it should be a huge problem.  That said, there's no reason to really go out on a limb in the description.  It's not like you're required to do the research you propose, so I would be more inclined to suggest doing a mix of problems from the new and old areas.  Also, talk to your recommenders about it.  If they're enthusiastic, it will really help.
I wouldn't over think this stuff.  It's just impossible to know what people will think.  I would concentrate more on what you think is best for you.  
You should discuss anything cool you've done with educational outreach.  If it was in another country, I don't think it's a problem at all.