I'll jump in, but these are just my opinions - others may have more or better information.

All violets, pansies, and violas are members of the genus Viola so yes they are all related. I imagine there are some that interbreed freely whereas some stay true. I have heard of people having them seed around their yard but they never mentioned interbreeding as a problem, so I wouldn't worry too much about it. Around here the pansies don't seem to mix with the wild violets. Sometimes you see a bit of extra color in a wild Johnny Jump-Up but there is no way to know for sure how it got there.

I believe all of them are edible but that doesn't mean that all of them have great flavor. As far as I know, violets and pansies as a commercial crop do not require any harsh chemicals (like growth regulators) so there should be no specific chemicals to watch out for. But most plants offered commercially do get exposed to a higher amount of chemicals simply because they are placed too close to other plants that do require chemicals to look good for market.

I grow a lot of different types from seed but it is definitely the slow way to go - if you can find live plants for sale you will have a blooming specimen much quicker.

I grow a lot of violets famous for their aroma - not everyone can smell them. Its kinda like Freesia or even Paperwhites; some people love the smell while others hate it or cannot smell it at all.

You could try Viola Valley Wildflowers-this is my first year ordering from them-so far so good. I had a few questions that were answered promply, by email and they even returned a phone call!! (ordered viola blanda,pubescens,canadinsis,and pedata).
I've wanted to order from Canyon Creek but have been unable to get anyone to answer questions! May just have to take a chance and order anyway 'cause no-one stateside has the selection they have! -B