I wouldn't bother with "why I want to go to law school." It seems like you're looking for a cheap way to include a second PS in this instance. And I'm not sure about the part time thing. I don't think it needs an explaination, espcially since you have a career. I think it's assumed that if you're applying PT, and owrking, that you will continue to work through ls.
As for the low gpa, if it's significantly lower than all your other semesters, or you have a legit reason (my dad was diagnosed with cancer, I was ill, etc) then I think that's definitely a good addenda to include. But if it's like a tenth a point lower than what your "normal" is, I'd be tempted to say that an addenda in that case would be frivilous.