1. naturalization don't have detailed income guide line as for GC.
2. as an extreme example, for a homemaker wife in Cat IR, even she don't have income she still can get citizenship.
3. good moral is a very broad concept.
4. file Tax return is a very important action show good moral.
5. IRS read your tax return mainly by computer. if USCIS choose to read your tax return, it would be an experienced officer with your other information on his desk or in his computer.
5. agree that USCIS independent make decision and judgement. the officer interview you and review your case has great power about your case.
6. other department and law enforcements actions such as Police, court, IRS... will definite hurt your case. but no those action not means you are 100% OK