Karl, The mineral spirits is used for two reasons. One, it thins the oil to allow the mixture to flow easier into and around the project and the fret holes. Second, It allows the oil to dry faster. When I used straight linseed oil, I would have to wait four to five days before the topcoat. Now that I use the mixture, I can topcoat the next day.
As far as the spray topcoat, it works best if you angle the spray so it sprays into the fret hole as it is coating the top. spray from all four sides. Once you have a couple of coats, turn it over and spray from the other side. This usually works foe me. Its best to spray two light coats rather then one heavy coats.