As a fan of the show, I would of course advise against skipping even the MotW eps and watch every episode, but if I were a newcomer to a show that's currently on its eighth season, I'd probably understand where you and your wife are coming from.
I would recommend you visit the Supernatural Wiki
, which features detailed summaries for every episode. Just do a search for "Season One," "Season Two," etc., and you'll be taken to pages with each season's episodes.
It's a little late tonight, so I don't have time for it now, but if you're having problems I can try to put together a list of the "mythology episodes" later. Although as the show ages the divide between standalone MotW eps and eps that contribute to the overall arc narrows. This is especially the case in the fourth and fifth seasons.