No, I am not a professional, just a volunteer been answering these questions for over 12 years. (This is a user-to-user forum). (You can hover your mouse over a user name to see their history.)
Yes, you can always convert a Traditional IRA to a Roth and pay the tax on the Traditional IRA distribution. That is just a normal conversion.
The whole point of a "Backdoor Roth" is to do that without paying additional tax on the conversion (the non-deductible after-tax Traditional IRA contribution offsets the tax on the conversion). When you convert a mixture of before and after-tax Traditional IRA then the before tax amount is taxable.
As said above, you can never only convert the after-tax money if there is any year end balance in any Traditional IRA - that defeats the purpose of a Backdoor Roth.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**