Welcome to the MC and the bulletin board.
The itchy "rush" feeling you got from taking niacin and drinking the lemonade was sunburn and other radiation appearing and then burning off. Eventually, that itchy "rush" feeling will disappear. It's actually a type of healing crisis where the symptom appears briefly before being completely handled by the body.
As regards high blood pressure and the salt water, if you've been ordered not to ingest any salt by your licensed health care practitioner, I would follow his orders.
However, if you're just avoiding salt because you've read that salt raises blood pressure, you might be interested to know that I don't know of anyone whose blood pressure has increased with the Master Cleanse, but I am aware of a couple people who specifically mentioned their blood pressure went down as a result of the Master Cleanse and kept their blood pressure low by eating a predominantly raw vegan diet.