It's an extremely personal decision most likely made between the golfer, his parents, his swing coach from home (if he has one), and his college coach. It all just depends on what they think the golfer can gain from being in the program balanced against what they think he could gain with the ability to devote 100% of his time to practicing (which is a huge advantage, as opposed to having to go to class, etc.). That being said, I believe Corning will be back next season. He's a hometown boy who I think feels like he gains a lot from having Penley and Buts as coaches.
You're a pro by simply saying so. To get a pga tour card you pay a certain fee (costly) to go to the Q school in November which is held in a number of sites around the country. The top qualifiers from those sites go Florida for a six round tournament. The top 25 get cards to the pga tour and the nezxt 25 get cards to the Nationwide tour. The top 25 money winners on the Nationwide tour each year move up to the PGA tour.