Not a bad idea to get the proof of time. Tough to choose though, between having a non-Disney race as your first, but a better corral, vs having your first race be Disney but starting in the back. And, just so you're aware, unless you're in the front of the first corral you'll still run into crowds in the first mile or two, lol. I'm not super duper speedy but I went and found a pic where I can see my race bib and it looks like I was in corral B, and I was still dodging and jigging around people for the first couple miles. The benefit to this, of course, is that it makes it pretty impossible to fall into the cardinal newbie race mistake - starting out way too fast and burning out
Either way though, you'll have a blast!
And that's good to hear about DL 2020. I'd heard they had been cancelled due to clashes with the city of Anaheim, so hopefully all of the construction being finished will put an end to that.