Jon Niese knows he’s not everyone’s first choice to be the Mets’ starting pitcher on April 1. “Johan [Santana] wants that Opening Day spot, so he’s working hard for it,” Niese said. “It’s his spot. If it changes, I guess I’ll welcome the opportunity.” Santana’s availability for that first game is still very much in doubt, but after the Mets got involved in a public spat with their most expensive player, it now seems both the team and its ace are confident he is going in the right direction. He may, in fact, make a Grapefruit League start as soon as next week. “He’s scheduled to get on the mound in about seven days,” Terry Collins said. “I think he’s gonna make that. All signs are pointing that he’s gonna be ready.” Even if Santana gets back on the mound in a week, it would be a long shot for him to pitch the opener. Santana wanted to pitch for his national team, but the Mets preferred he didn’t — and as Collins pointed out, he wouldn’t have been ready, physically, anyway.