It just takes time, lots of it!
I have only needed the aux once and that was off Coll not a million miles from you. I ended up coming back to Oban on just the aux then partial power on the main engine. I would have got home on the aux eventually as it was around 30miles.
After a few miles I managed to repair the main motor enough to get 16-17knots and we came home on this but the aux got us into the sound of Mull and would have got us home despite the distance.
Operating out there I would probably be thinking of twins just for the remoteness but at least an aux will get you back to shore instead of floating off to the US, I mean its not as it you can anchor out there