These are all hard questions. Dizziness is usually related to low blood pressure. During a Water Fast your pressure will drop. So always be careful when you stand up.
I still think you are magnesium deficient because of arm and leg pain.
There is no way you can know your blood chemistry without a blood test. I believe in fasting and listening to your body, your body signals you all the time. I do have a blood Sugar meter and blood pressure monitor that I use during my fasts.
About small light and speckles in your eyes can only be tied to low blood circulation. I have never experienced that so hard to tell.
Nausea is a common symptom during a fast. Although I must say that I have never experienced that either. Eating a good diet before a fast does help with your fast.