The main thing that will keep you from entering into ketosis is ingesting carbohydrates. Carbs break down into glucose, the basic fuel for the body. As long as the body has access to a form of glucose it will not completely kick into ketosis, but it may continue to break down muscle. The probiotic you mentioned would probably not cause you problems.

Water fasting is extremely difficult, and stress will make it more difficult. If you feel that you must eat something, try sticking with raw fruits and vegetables. They are low calorie, provide essential nutrients, have fiber to help you feel full, and their more complex nature will not break down and release sugar in your system. IF you can keep some apples, oranges, watermelon and grapes handy you can have a sweet treat without destroying your diet. As you continue to do fasting days and eat a healthy raw diet your body will gently cleanse and water fasting will become easier. Perhaps most importantly for you, you will continue to lose weight instead of having it yo-yo back and forth.