Fixed it. Apparently I hadn't properly cleared the IRA contribution entry I made following the "Backdoor Roth conversion" article instructions, so I was still getting the messages saying my contribution was not allowed, and the 6% penalty for that was still showing in my total taxes. First, I went into Forms, deleted the 1099-R for the trad IRA distribution, and deleted the contribution amount showing on the IRA Contributions Worksheet. Then I went back to Step by Step/Federal Taxes/Wages and Income/Retirement Plans and Social Security/IRA, 401(k), Pension Plan Withdrawals and re-entered the 1099-R for the distribution. I checked "No" for the next two questions. For "What did you do with the money from XXXX?", I checked "moved the money to another retirement account..." and "did a combination of rolling over, converting, or cashing out the money", then entered the full amount of the IRA distribution in the box for "Amount converted to Roth IRA account". Return is now good to go with a small refund instead of a $2.4K payment owed.
The "Backdoor Roth conversion" article cost me a lot of time and hassle, and I still can't find instructions for my particular situation, which again seems like one that would be pretty common for retirees. Y'all oughta fix this, Intwit!
