Then what could he have done better?

Are you saying that he should have given up? To veto would have been to give up, to actually cave. At least he's fighting back, even if it doesn't meet your approval. It's not "mental gymnastics", it's actual thought and reasoning.

Trump isn't an idealist, he's a pragmatist. Don't pretend that he's in a position that's strong enough to live up to your ideals in this scenario. There's more at stake than your sense of pride, and his.