Duart Castle is only a few miles from Craignure (plus a 4 mile entrance) but I think there are better places to go, given your time frame. Tobermory is approximately a 45 minute drive from Craignure. The first half or so has good double-track roads but you will be going much slower the last third of the drive when it changes to single-track so count on a full 45 minutes at least.
Tobermory is charming and there is a lot you can see and do there. Just park in one of the free car lots on the harbor and walk down through the shops and restaurant that line it. Bring your camera as it's very photogenic. You can visit the Isle of Mull Cheese Shop (Sgriob-ruadh Farm), which is about a mile from the harbor. There are self-guided tours to meet the farm animals and learn about traditional cheese making. There is also a cafe, and a farm shop.
The Tobermory Bakery is wonderful - definitely stop in here for one of their delectable pastries and coffee or tea. Their Millionaire's Shortbread is incredible! There is a shop at the end of the harbor that is farthest from the ferry terminal that sells handmade chocolate with a tea room, and along the way are some very nice gift shops - I bought half my souvenirs in Mull (and I'm picky about stuff like that). Lots of nice restaurants, too. There is also a catch-and-release aquarium that is interesting, too.
After exploring Tobermory, you could head towards Calgary Beach, which is about a 30+ minute drive - probably more like 40 - from Tobermory. It's very narrow and the single track roads run through the mountains with some switchbacks, so you will need to go slow through much of it. I did not make it there because of my fear of heights, but others have been and it's well worth a visit to see the beautiful beach.
I would not even attempt Fionnphort or Iona - it will take you close to 2 hours to get there from Tobermory, and even more time if you add in Fidden Beach and Calgory Beach. You're looking at probably 4 1/2+ hours of driving. The scenery is lovely but the driving will be tiring. I would just skip Fionnphort and Fidden Beach and concentrate on Calgary and Tobermory.