Okay Starchild didn't appear out of nowhere. He was on a previously unexplored part of the citadel. For the purposes of the plot they really both appeared out of nowhere their physical locations aside. The point is that virgil was indeed unexpected just as the starchild was. The starchild doesn't just APPEAR hes been behind the scenes running the reapers. Your ability to swallow incredulity should not have any bearing on wether or not you think the star child did or didn't appear out of nowhere.
He didn't take shepard to the buttons. Shepard had to walk and move over to complete the action. In the end Shepard made the decisions and decided the game. Not the star kid. If the star kid was a real deus ex machina than he would have appeared said some dribble and the game would be over. No Shepard painful moving to pick an ending scene. In fact he wouldn't leave you a decision at all. Zeus in the old greek plays would take it UPON HIMSELF to act for example. The star child still leaves it up to you.