You will not see a zero balance due until you enter a receipt of the full amount receivable. This has been explained above by both @Brucanna and me.
Yes, you do. But your post said you were applying a tax rate of 14.974%.
No, the cash account must always be involved, because that is the only way to receive money. However, if you are always being paid COD, you can bypass the sales invoice. See this new Guide:
Note that when selling without a sales invoice, you must select your tax code on the receipt, because the tax has not yet been calculated and applied. In this case, however, the amount you enter must be tax-inclusive, because the cash receipt form does not have the ability to add tax, only to show how much of the total is tax.