The plan had been to proceed to a second round of interviews. But once Mickey Callaway presented his case to become the Mets’ 21st manager, team brass shifted focus. Their mission no longer was whom they would hire but how they could persuade him to take their offer. After a three-hour Manhattan lunch with owner Fred Wilpon, general manager Sandy Alderson said Callaway was “too exhausted to say anything but yes.” “I am the most excited guy in the world right now to be here and to be able to work with these people,” Callaway said Monday at a news conference in which he was officially announced as the Mets’ manager. The Mets began with more than 30 candidates. Only six were interviewed, including a previously undisclosed hopeful, broadcaster and former player Mark DeRosa. But Callaway, 42, emerged as the winner, his candidacy bolstered by his success as Indians pitching coach and what Alderson called “a contemporary thinker in terms of strategy.” “I’m probably the most excited guy you’re going to see in a long time,” said Callaway, who showed some of the qualities that made him the replacement for Terry Collins. Callaway has never managed at any professional level, yet he handled his introduction with wit and charm.