Cluck, I have purchased all but one of Mitches tutorials, but only use them for reference just now. Later, I will actually give them a go. I am glad to hear you are using his basic course it will do you well to practice a lot, especially getting the basic strokes down. When you go to his seminar, I think you will find he EXPECTS you to know these and how to do them well. Otherwise, you will be wasting his time and your money. Perhaps you should read his sylsabus or send him a quick note first to confirm my assumption. I have been wrone quite sometimes so you might want to check to be sure.
As far as skulls go, there are some airbrush artists out there that are quite good at doing skulls, and in fact, make a living at it. So don;t give up so easily. Actually, those that get quite good at airbrushing skulls find out that they can actually do portraits as well. It seems to be a matter of being able to control the airbrush and duplicating what you see. So keep at it, it will only get better. I wish I had a dollar for every pair of eyes I have painted and discarded just trying to get the movements down. So we all strugle at the same thing you are experiencing right now. Just keep at it and it will just all of a sudden happen.
As far as photo realistic is concerned I am trying to move from this to a more personal style of drawing and airbrushing. I have come to the conclusion that even though this is popular at the present time, I don;t want to continue to be just a human copy machine and discover what I have created can be done in a matter of seconds with most any photo manipulative program or a copy machine. Try to be who you are and not something you are not. You might find out later you are the next Rembrandt or Picasso....
Remember: If it's cheaper, it is.