some years go I did a day trip to Mull and Staffa from a place near Oban. For me it was a wonderful day - but I have to admit, it was a lot of driving, first half to Fionnport completely in rain and fog. The road back with a detour thru the mountains was wonderful (including sunlit rocks and some highland cattle). And I had the luck to just get sunbeams when we landed on Staffa. Perhaps I should remark I had my own car with me.

Downside - I didn't do Iona, after I was simply overwhelmed with Staffa. ;)

So, if you can restrict yourself to one or two highlights, it's doable. If you'd rather experience more of Mull and Iona it might be better to stay on the island. Otoh - if it's raining you really see next to nothing, so pre-booking can be "dangerous".

Baerbel (near Stuttgart)