I agree with Shane. I'd probably just finish out your last two visits. UHC/optum accepts programs like weight watchers for supervised diet visits which wouldn't be endorsing surgery, so I see no difference with her either (if that makes sense?). Her role is to document your weight and guide your nutrition and exercise goals.
If you are set against seeing her again, I know my nurse coordinator with optum said I could see my PCP for 3 months then see a nutritionist for the next 3 months. This would satisfy their 6 month requirements even though it wouldn't be with the same provider. The 3 and 3 month is just an example, it could be any combination. I just received my approval through UHC Optum, let me know if you have any other questions and good luck with the approval process!
I would call your surgeons office for more clarification. Part of my 2 week preop diet is no caffeine, so real coffee is a no-no. The no caffeine rule also extends for me 4 weeks postop. It's gonna be a really difficult 6 weeks... My allowed clear liquids are water, crystal light, G2/vitamin water, broth, sugar free gelatin, sugar free popsicles, and decaf coffee or tea. Clear liquids 24 hours before surgery for my surgeons requirements.