For the second part of your question order of merit is not that important in determining what you select as most people get their first choice, however it defenitly helps your odds to have a higher order of merit. Very few people get lower than their second choice, partly due to different people being interested in different things so the service selections kind of sort themselves out. In addition, some billets are affected by other factors, like marines are influenced by their leatherneck performance and interviews, competition for air billets gets reduced by medical screenings and small communities like SEALs or medical often have almost nothing to do with order of merit.