a) Roth conversions are done in a particular year, not for a particular year. If your 2018 tax return already reports the $5,500 traditional IRA contribution that resulted from the recharacterization of the Roth IRA contribution, either as deductible on Schedule 1 line 32 or as nondeductible on Form 8606 line 1, there is no need to do anything with your 2018 tax return.
b) Already answered: "Make sure that you go back to the Traditional and Roth IRA Contributions section in 2019 TurboTax and, just as you did (or should have done) for your 2018 contribution in 2018 TurboTax, enter the $6,000 Roth IRA contribution, indicate that you recharacterized $6,000, then indicate, if TurboTax asks, that the resulting traditional IRA contribution is to be nondeductible. TurboTax will include this on line 1 of your 2019 Form 8606 to be added to the $5,500 on line 2."
3) If your 2018 tax return does not show the traditional IRA contribution on either Schedule 1 line 32 or Form 8606 line 1, or your 2018 tax return shows the traditional IRA contribution as a deduction on Schedule 1 line 32 but you wish it to be a nondeductible contribution instead, you must amend your 2018 tax return. While it's not entirely necessary to do the amendment first, it's better to do it first or at least prepare it at the same time since the amount that appears on your 2019 Form 8606 line 2 must be the same as the amount on line 14 of your 2018 Form 8606. Any change to your 2018 tax return must be filed by mailing; it cannot be e-filed. The IRS initial checking of your 2019 tax return will likely not include checking against your 2018 Form 8606 which would delay any refund with respect to your 2019 tax return, but may later detect a missing Form 8606 to corroborate the amount you will have shown on line 2 of your 2019 Form 8606 and inform you of a balance due.
