1. There will be no change in tax liability. A Roth conversion is not subject to any early-distribution penalty regardless of your age, so, assuming that you have no basis in nondeductible traditional IRA contributions and your traditional IRA contribution for 2020 made in 2021 is deductible, the deductible traditional IRA contribution will offset the increase in income from the Roth conversion and there will be no net change in AGI.
2. No. A Roth contribution adds to your overall retirement savings without changing your AGI. A Roth conversion results in no change to the amount of your retirement savings but adds the taxable amount to your AGI. The benefit of a Roth conversion is that you pay the taxes now on what is presently deferred income in the traditional IRA for the privilege of getting tax-free growth in the Roth IRA instead of tax-deferred growth in the traditional IRA.
Regarding #2, if you were instead asking if there is any difference between just making a Roth IRA contribution and doing what you proposed in question 1, there would be no difference given the assumptions I expressed. However, some people could be ineligible to make a Roth IRA contribution due to AGI being too high, so what you proposed in question 1 can be a way to get money into a Roth IRA that cannot be contributed directly to the Roth IRA.
