Phew. Lots of good questions... here goes.
1. Assess my weaknesses. How will those impact me? Are they biggies or things I can explain with little harm? Any other ones?
Yes, the job hopping does appear a bit flighty, but you explain it well. As long as you represent it as such (financial need then moved to real areas of interest) as well as outlines your career goals crisply (i.e., send the message that you have focus and aren't just a dilettante) and it should be fine.
The post-graduation Cs aren't great, but since it's not part of your core GPA aren't a show-stopper either. I wouldn't spend any time trying to explain these away unless asked.
2. Because of my irregular background, it's hard for me to point out specific accomplishments as a teacher, and it's also hard to demonstrate leadership / teamwork experience outside of the classroom. Any ideas here?
What do you mean you can't point to specific accomplishments as a teacher? I have a close friend who was a teacher before b-school it was apparent that although he might not launched a multi-millilion dollar product, he could point to the impact on and innovative measures he undertook to educate his students. That's the last time you should imply that and should spend some time detailing how and why you had an impact as a teacher or you'll have a tough time selling yourself. Don't worry about outside the classroom until you get inside nailed down.
3. Thoughts on applying to same school as girlfriend, pros/cons / process of applying as couple
I applied same time as my girlfriend (now wife). We applied to the same schools, but she ended up going to Michigan and I to HBS. If you both get into the same school(s) and both want to attend, then it can be a great experience. A lot of times the chips don't fall that way, so be prepared for that. You'll also have to be prepared for what you'll do if she wants to go to one school and you another. There will likely be some tension as responses come back in, etc. At the end of the day, there's your relationship and your career aspirations. Yes, there's overlap, but you should never make any decision today that we'll make you resent her or yourself tomorrow -- and vice versa from her perspective.
4. In my essays, how much detail should I go into reg. my exp working at an investment bank and marketing? Are they even relevant at all? I have written a number of essays, and all my ex. of leadership/teamwork/etc. are all coming from my comm service (board membership) and creating/teaching classes (teaching exp.)
Didn't you say you didn't have much outside of the classroom? Board membership seems relevant to me. Anyway, you might want to position yourself as a teacher who's passionate about non-profit (future goals), but who completely understands the relevance of the for-profit arena (see past experience). Thus, the majority of your app will focus on teaching and community service with a dash of the for-profit stuff. To more directly, answer your question it's okay if you don't feel like the banking / marketing stuff isn't as relevant. Don't force it.
5. Eval chances at schools.
I think you'd like Michigan. have you looked at Berkeley? I think you'll be competitive if you just take a quick confidence shower and nail down your teaching accomplishments.