R&S, don't be nervous. If you can run a 5K (no matter the speed) you are very close to being able to knock out the 10K. I would suggest that you start on week 9, and see how it goes, if you feel like backing up a week or two to feel comfortable, that's fine, as well. I wouldn't worry about getting in just two runs per week, you'll still gain the benefits if you keep on a scheduled. I think you'll be good advancing each week every nine days (or whatever your schedule allows). You may not complete the 10K program before the end of March, but you'll be close, and when you get to race day, you'll be surprised how ready your body is to make it the entire distance.
I breath through my mouth almost exclusively when I run. I've always felt like I can't not get a deep enough breathe and don't get enough oxygen when I breath through my nose.
Best of luck, keep us updated on your progress.