For what it's worth, I figured out how to "fix" my amended return. In form view, I looked at the 1040-X form, and noticed that line 1 column C (supposedly my corrected AGI) was exactly the AGI on my 2023 return. Line 1 column A (which was supposed to be my original AGI) was wrong. It was $1,115 more than it was on my 2023 1040. Fortunately, the software let me change the Line 1, column A value to the correct AGI, and then I was on my way to things making sense. I also had to correct Line 6 column A (my original tax), line 11 column A (my original total tax), and box A between lines 17 and 18 (my original overpayment).
After doing all that, the Federal Refund was zeroed out, and I was able to modify my FTC to include the carry back from this year and all was well.
I can't even begin to explain why the software decided to modify the AGI from my original return, but I hope this maybe helps someone else out there who tries to create an amended 2023 return and experiences the same dumpster fire that I did.
And shame on Turbo Tax support that assured me that Turbo Tax is like a master CPA, and that I should blindly trust whatever it does. I could have saved so much time if I had given up early on that call instead of trying to convince the support person that there was clearly a problem with the software.
