If on Jan 1 you redirect 100% of your salary to a 401k, then quit once you hit $19500 (the 401k limits), do you still get the whole 80k CG at 0% rate because the 401k was a "top line" (before AGI) deduction or is it based on total income?
It's based on taxable income. There is also standard deduction ($25k if married) so technically I guess you can make close to $50k and still pay 0% on CG.
Is health insurance paid for by CG profits deductible? Since Health insurance is another top line deduction can you work an extra few weeks for that money and still get the 80k 0% CG tax rate?
Non-employer health plan payments are not tax-deductible.