First question, well, they do say yes for "long" fasts, that is over 10 days, but I think that is probably a good idea for run of the mill people with no supervision.
If, on the other hand, you are fairly intelligent, and know the difference between fasting and "fasting apart from xxx each morning", and visit this forum regularly, then I dare say you can do longer fasts without the need for a doctor.
I did a 27 day fast, was very happy with the advice I got here, and saw no point in spending hundreds a week seeing a doctor just for him to tell me "keep drinking water".
Besides, I did my research, and found that no other animals except humans consult a medical expert in their species when fasting :). Neither did my ancestors have doctors when they fasted after a bad harvest, yet they seemed to survive (well I am here anyway).
So I would say ask here, if you are in good health and the fast goes well, and you are happy with it, don't be terrified about crossing the 10 day threshold if a man in a white coat is not readily visible.
On the second question, yes, drink when thirsty. The thirst mechanism is there to tell us to drink. Use that, it's much better than what you read in books! :)