Bummed to have this happen, so shitty to have such a key issue be totally outside Glenn/Nathan/Milton's control, especially since you've worked so hard to deal with the visa process and do everything in your power to avoid these issues. I suspect there will be a chapter in Glenn's eventual book dedicated to the USCIS, I suspect it will contain a lot of profanity, and I suspect it will be an awesome read.
As to using the now open timeslot:
Step 1: Ask Blind Guardian how much it would cost to get them to add Nightfall in Middle Earth front to back to their set.
Step 2: Give us a PayPal address and let nature take its course.
I kid, I know, that's wholly unrealistic on this short of notice (or at least step 1 is, pretty sure I'm not the only one who'd help with step 2), but I actually wouldn't be opposed to just giving Pyramaze, Circus Maximus, and Blind Guardian extra time. They still stand as a strong lineup and considering BG always practices almost two entire sets worth of music before a tour, they can probably make use of it. Regardless, whether you add another band or not, it'll be a great night and weekend of music.