That lawnmower damage is nothing new to that tree. So I don't think we can say that is the "immediate cause". Certainly I'd say it is likely a contributing factor to what we see.
I suspect the black dots are phytophthora bleeding canker? But maybe Botryosphaeria? Or maybe an insect? I need to cut a chunk of bark away to get a better idea. Destructive sampling isn't going to cause more harm to this tree, but the client may not be on board with it. If I see galleries under those black spots, we'll have a better indication that it is insect damage (but that alone doesn't rule out the fungal pathogens). Otherwise, we can send to a lab for diagnosis.
Is that, then, the cause of the dieback? Possibly. Neither of those show up overnight. Verticillium could still be at play. Is that just me wanting my original guess to be right? Possibly. An otherwise healthy tree can live with Verticillium. However, once it is looses vigor, the wilt fungus plugs vascular tissue more rapidly than the tree can respond - especially if it has been hot and dry. Having said that, I'm a little less keen on verticillium and more so on phytophthora after those latest pics.