I have one more point to make about the train vs tours. On the train your travel the same countryside but have more flexibility than you do on a bus. It's not "wasting a day" when you take the train to Oban. This is a beautiful train ride and one to be savored. Also, please someone, provide a link to a one day tour from Edinburgh that includes a visit to Iona. I don't think it's possible, but would be glad to be proved wrong for this traveler.
So in the end, you need to decide what you style of travel is. I agree that your time is tight, and I might not try to get to Iona, but I figure you have reasons for your travel dreams. You can do it.