I think it's less about race and more about embarrassment. He seems to have deep-seated psychological issues that are being triggered by having been mocked at the correspondent's dinner. I'm not sure how he felt about Obama before, but he has become obsessed with fighting back and undoing everything Obama did just on principle -- regardless of whether he otherwise likes or agrees with the policy.
No. The dossier has neither been discredited nor substantiated. It was Republican opposition research (I don't think we know what candidate or group funded it) and then was later picked up by someone connected to Hillary Clinton after the Republican primary was over.
AFAIK, there's no more or less reason to believe it now than there was when it first came out.
