The explanation is for the IRS and will be included with your e-filed tax return. The IRS wants to have a record of the reason that line 2 of the Form 8606 does not match line 14 of your mother's last last previously filed Form 8606 (if any).
"In the Amount box I typed in my Dad’s basis from his 2023 F8606, Line 14."
It should actually be from line 14 of your father's 2024 Form 8606, but if he made no nondeductible traditional IRA contributions for 2024, it should be the same as line 14 of his 2023 Form 8606.
"Will this explanation suffice?: “I acquired the additional basis when my spouse passed and I assumed his IRA as my own.”
Sounds reasonable. I doubt that anyone will actually read it, but it's there if they do.
"I entered in the IRA values as of 12/31/24 for my Mom’s 3 IRAs (her IRA she’s had for many years, one she inherited when her brother passed in 2011, and the one she just assumed from my Dad."
The value of the IRA your mother inherited from her brother is not to be included. Only include the values of her own IRAs.
"'None of these plans failed to withdraw the RMD.' Is this the correct choice?"
Correct. Upon his death, responsibility to complete the RMD moved to your mother.
It's correct that your dad's information is still present on the IRA Information Worksheet. It would have been needed if he had taken some distribution.
I would not count on information automatically from the Spouse side to the Taxpayer side on the 2025 IRA Information Worksheet when you transfer in the 2024 tax file. If it doesn't, TurboTax should flag an error for it still being on the Spouse side and you can then move it yourself, most easily done in forms mode of the desktop version of TurboTax. You'll want to check your entire tax return for similar situations on any form or worksheet that has both sides.
