1) If you only select one file at a time you can uncheck the box for the cover art. I don't think there is a way to prevent it when doing multiple files.
2) No. I don't know if J River is planning to implement this or not.
I have all my movies, prescraped with ember media manager for use in other software. If i hit the get tv & movie info button in JRiver, exactly what I am going to get, I too don't want JRiver to overwrite what I have but if it will give me more than just the poster image it is displaying out of my movie folders than great, like actor, imdb rating or the like.
Any response would be appreciated.
TC