This. I feel like anything else (given 2020 around the corner) is just sensationalist

My own feelings about Trump set aside, I can't imagine he'll prevail.

On this exact matter, he probably would. Lawsuits telling the executive that they must use their discretion in a certain manner, thus essentially stripping them of that explicitly-written discretion, probably don't have a leg to stand on and only judicial activism can save them. It's a silly idea, considered philosophically rather than in the heated political moment. What you'll get is something like what OrangeMango said, where they use other methods to enforce their will other than a particular authority for the EPA to carve out exemptions to a particular law. They've got all sorts of other tools.

Even so, think about the damage he will do along the way.

In this case, not much. The automakers don't want the rules to change and are extremely unlikely to start retooling factories since this is a temporary situation.