#1
Well, what's the best approach for getting it properly back up to tension?

...and by the way- I wanted to unscrew the nut completely so I could lubricate the threads, but it seemed to get tight when I tried backing it out, so I left it alone... What might the reason be?
#2
Why was it loosened in the first place? I'd take it in gradual stages with a full turn at first, then leaving it for around two hours, then making about a 2/3 turn. Then leave it an hour or so, followed by a slightly smaller turn and shorter resting period - decrease these each time, ad infinitum...

Restring after the first full turn or so and tune up to pitch (to gauge how much more you need to compensate by) then slacken again.
#3
f that man. if you even need to ask how to fix it, take it to a repair shop and have them do it. you could f*ck your guitar up for good if you dont know what your doing. its worth the money to have a certified repair guy do it for you. theres a lot more than just turning the key til the tension is even kinda thing.
Jackson RR Professional, early 90's japanese model, with EMG's

Digitech GNX3000

Crate GX 212
#4
I'm not gonna take it to a shop. Might as well learn these things eventually.

It was loosened because I switched out the tuners. See, I had to drill some holes for this little jiggy on the tuner housing, but the drill bit I had broke, so I couldn't get it done for a couple of days until I got the opportunity to get a new one, so I just loosened it to save it from warping. See, it was set up for 11s, (and I did that myself, so I am fairly sure I'll be able to tighten it back up without borking it all up) so I figured it'd be best not to let it stand stringless under tension...