1) Yes, check tags. They would be the reason the incorrect film is selected. If you are manually doing the Movie lookup, you should be able to change the search criteria so that MC finds the correct movie, and if it returns more than one result, pick the correct one from the list.
2) If you only have one zone, probably the default if you haven't set up anything else, and are playing a TV show, then start playing a movie instead, the TV show should stop playing and the movie should play correctly. That is what my system does. I just tested it again, but by double clicking on the shows rather than using a remote control play button, but it should act the same regardless.
If you have set up multiple zones, then follow Arindelle's advice.