If as advertised the National Retail Sales Tax is revenue neutral then initially it will have no affect on the net amount of money available for the purchase of goods or to invest. Taxing consumption while not taxing investments should lead to a shift away from consumption and towards investment. With the rebate for necessities, necessities will have a net decrease in cost because taxes will no longer be included in the price. Beyond that you can argue that some prices will have a net increase or decrease depending on the difference in the percentage of the cost due to taxes.
Every thing I have seen or read says interest is exempt so where did you find different info?
Gross payment tax in some ways is typical Government in trying to make numbers look better but it is a true representation of the percentage of the payment that is spent to fund Government.
I prefer the National Retail Sales Tax because it will allow for the easy flow of Capital to where it can be most efficiently used. Current income and Capital Gains taxes discourage the sale of assets and reinvestment because the new investment has to justify the loss due to taxes paid.