sacrificing GPA to graduate sooner is NOT a good idea. If you look throuout this thread you'll see that GPA and LSAT are pretty much all that matter. In fact, many people here press to take tame off b/t UG and LS. Law schools do not care how long it takes you to graduate (I'm saying this as a soon to be young applicant myself).
I,m not really sure about the 12 cr question but it I'd be bored if I only did that much. If you feel you should do it though, find something to fill the extra time other than partying. Job, voluteering, campaigning etc.