Yes, you can amend your return before filing your 2019 return.  If the IRS letter reads that they corrected your return and you agree with the amount of tax due you could just pay the tax without amending your return since they really amended it for you. 

Sure ... but you must use the 2018 amendment program AND mail in the amended return.  However, if the IRS has already corrected your error you may find amending the return will do nothing for you except update the 2018 return for the transfer to the 2019 return. 

first amend :

https://ttlc.intuit.com/community/amending/help/how-do-i-amend-a-return-in-turbotax/00/27579

then transfer that file : 

https://ttlc.intuit.com/questions/1900883

You can amend the 2018 tax return before filing the 2019 tax return.  However, it will not make any difference to your 2019 tax return if you do amend the 2018 return or not before filing the 2019 return.  Unless you have some carry forward items on the amended 2018 return (uncommon).  The payment of federal taxes owed for a prior tax year is not entered on a federal tax return.

You use Form 1040X to amend the 2018 tax return.  The amended return can only be printed, signed, dated and mailed by you to the IRS, it cannot be e-filed.  The IRS taxes about 4 months to process the amended tax return.

See this TurboTax support FAQ for amending a 2018 tax return - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-a-return-in-turbotax/01/27579