When you apply to OCS, you apply to specific programs. So, in your application, you'll list your first choice as SNA. Your application will go to the selection boards for each of your (at most) three choices concurrently. If your first choice recommends you, then you should be good to go unless you have a medical issue. So, it goes something like this: apply for SNA, get professionally recommended by the SNA board, get medically cleared by MEPS, have all remaining docs submitted by your recruiter, receive selection notification for a specific OCS date, show up for OCS, get flight physical. So long as you pass your flight physical and graduate from OCS, you'll start flight training.
As for the ASTB, don't sweat it. Your recruiter will let you know whether your scores are competitive or not. The test is not terribly difficult, and if you study the Marine Corps ASTB guide, you should have no problem getting a decent score.
Hope this helps.