Update for anyone who'd like to get an interview at CLT:

Per my post the other week the situation seemed pretty bleak; online scheduler showed no appointments available at any time in the future. Gave them a call and asked a few questions.

With respect to walk-ins at CLT, they do accept a limited number. While customs opens at 8am, the person I was talking to said something to the effect of, "If you really want to try it, I'd say don't be in line any time later than 630am. There are people in line as early as 430am. And once we take as many people as we can, that's that. Doesn't matter how far you drove, came from Raleigh, if you're going to call your congressman, whatever. When it's done it's done."

With regard to appointments, they are/were (at the time of the call) booked through the end of 2019. However, they suggested continuing to check online for openings, and that towards the end of this month (August '19 as I write this) some more openings should be becoming available, might even be some night time appointments available, but they just couldn't go live with that calendar just yet.

I kept checking the online schedule at least daily if not multiple times a day, and eventually some openings did appear! One was the same day and I couldn't take off work. Then some opened up end of September, which I'm currently slated for.

Moral of the story is keep checking, they do appear!