did you get a 1099-R for the Roth conversion? you would input that under Wages & Income / Retirement Plans section. Be sure to answer the interview questions after the 1099-R to specify that the money moved into a Roth. See following tho this is for backdoor Roth so it includes extra steps for the nondeductible contribution but the 1099-R steps should be similar https://ttlc.intuit.com/turbotax-support/en-us/help-article/retirement-benefits/enter-backdoor-roth-...
I think when amending a return you need to save your tax return file under a separate name before starting the process. See https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-federal-tax-return-curr...
Not sure your last question, don't you have a tax impact from this conversion? For 8606 if you just need to file the form because of an update to the basis say but no tax impact you can send in the 8606 rather than amend your return - see notes at the end of the backdoor Roth conversion article above. Not sure for other forms how that is handled.
