RE: Ultimately Sales/Revenue/Income = Amounts Billed/Paid by Customers, agree?
No. Sales and income are not the same thing. Similarly Bills and expenses are not the same thing.
Income includes things like Interest income, deposits for things other than sales, and so on - where many of these things are either not sales or not usually tracked as sales in QB.
Expenses include things like depreciation, petty cash, credit card charges and so on - none of which are usually tracked using Bills.