He's made a record of supporting coal and opposing the EPA. He's supported Keystone. He's voted against funding for Planned Parenthood. He's established enough of an independent record to win re-election, depending on who his opponent is. That's better than a minor role in a Trump administration.
Because he chained himself wrist-and-ankles to a party that vowed to destroy the coal industry. He is liable to get his head handed to him in the next election no matter what happens.
Yup. If he is a loyal trooper with Trump, he rehabilitates himself and gives himself a new lease on his career, plus a cabinet position on his resume!