If I waited to start my final research paper until right before the deadline, and then my grandma ends up in the hospital, it's not my grandmother's fault that I have to scramble over the last 24 hours. My hand wasn't forced. I was ill prepared and ready to adapt to when life happens.
He knew (or his campaign did) that Pence's deadline was coming up. His campaign knew that this was leaking like crazy to the media.
The problem is the Trump campaign continues to be in disarray and even without Corey, there is still heavy infighting and pulling of Trump in multiple directions. Trump can't make a decision. The VP choice is the first easy "presidential" decision someone makes, and the rollout is the first easy execution of a "policy push" that they make, too.
Nice was terrible, but this was already terrible before Nice and remains terrible during Nice when you should be able to be effective through Nice.