Oncologists expect a lot of questions, particularly at the firs time appointment. Most of the questions you have are usually covered in their "mini presentation" to you explaining what they recommend.
Can answer the biggest one for you though, if cancer is no longer there how is success measured? In short - they can't. We early BC patients do chemo as a risk prevention strategy. To remove any micro cells that aren't visible on imaging but may be there.
The diet question - will depend on how holistic the oncolgy department is. Your breast nurse is probably best place to start with that.
Exercise - the evidence indicates as much as physically tolerable, ideally something everyday, both during and after treatment. Improves side effects of chemo, mental health and long term risk reduction. Ive not exercised maybe 6 days out last 3.5 months of chemo, doing at a minimum 3km walk with my dog, some days can run 7km no problem, others forget about it.
There's a really good book which might help the diet/exercise/immunity stuff from a "beginner" perspective called the complete guide to breast cancer by Dr Liz O'Riordan, check your local library