How often is it ok to go back to Phase I?

Pre-SBD, it was a huge struggle to lose each and every pound. I was considering WLS! I'm worried that when I enter Phase II, my weight loss will stall again.

Is 2 weeks of Phase I, 6 weeks of Phase II, 2 weeks of Phase I, etc. ok? I understand the concept behind SBD, to teach healthy eating habits. But is it ok to zig zag back and forth while trying to shed the pounds, then move to Phase III when I hit my goal?

Phase I is ONLY to get rid of carb cravings and is to be done for just the initial two weeks. After that your body needs the nutrients available in the less-restricted of Phase II. Just add fruits and starches very gradually. If your weightloss really stalls, you may need to check your calories. In Phase II most people lose 1 to 2 pounds a week, a healthy loss.
I hope this helps and good going on your loss so far!

No, absolutely not. Phase I is not a balanced way of eating. You need complex carbs and fruit.