New cyclocross bike. Been ridden 100 miles. Talking rear wheel problem here.

Was putting second set of wheels on (Campy Vento 2006, other wheels are Vento 2005) and brakes rubbed the rim on what appeared to be both sides. I figured the new rim was a little wider, so I was tightening both sides of the assembly what I though was equally (to increase the spring tension to move the brakes away from the rim) via the small phillips head screw you use to center the brakes

Now spring tension is gone on left, so whole assembly is shifted to the right and left brake pad firmly fixed against the rim . Small phillips head screw you use to center the left arm came all way loose and cannot seem to retighten it, with brakes released at front or not. I try to rescrew in this screw, it starts to engage, but then just doesn't catch. I tried pulling the center of the brake assembly (that's above the wheel) over to the center while screwing in the screw, but that did not help. No individual piece physically looks broken.

What have I done?
How do I fix it?

Thanks for your help.