The bolt on my bathroom sink trap won't tighten

What a day for an amateur plumber. My bathroom sink is plugged up, and it looked like a good DIY project. The suction plunger did nothing, the auger did nothing, so I decide to undo the trap and clean it from there.

I got the front bolt on my trap partially undone, enough so that the backed up sink spilled out over the bolt and into my drip bucket. What's happening now is that I can't get that bolt re-tightened. I've cranked that sucker around and around at least 400 times (which was when I called it quits and turned to the experts - i.e. you all!). I never got the bolt fully open nor did I get the trap open, but neither can I get it closed.

It's old, old, old galvanized plumbing, and going to be entirely replaced in 3-4 months (which encouraged my DIY experimenting on it... How badly can I screw it up if I'm getting it replaced soon?). Can this bolt get stripped so it just spins in place? Is there anything I can do to get this closed up?

Thanks for the help -

Jeff