The first week or so your body is going through many changes. Don't expect to feel better after 3 days! Give it some time. Although I can't answer your questions about cancer, I could almost guarantee that early humans did not subsist on mostly fruit. Other than berries (which are relatively low carb and tend to ripen in spring time), most other fruits ripen after the Summer Solstice. So there is a window of time in which fruit would have been available. Is it a coincidence that the fruit (and most starchy vegetables) ripen at a time that early humans would have needed to add fat to their bodies to survive the winter? I give Nature more credit than that - nothing is a mere coincidence.
Try to suspend your disbelief and low carb for at least a month. Get your bloodwork checked before and after. I know my triglycerides dropped over 100 points in that amount of time.By the time you are up to 25 or 30 carbs a day, as long as you are choosing fiberous veggies as your carb source, you shouldn't have any problem with getting more than enough phytonutrients.