So it seems that I cannot use the existing unused basis from 2019 for 2025, and instead I need to do the calculations as if I correctly filed and claimed the basis each year, then use the new basis obtained this way for this year's taxes? I understand I can file amended returns for 2022 - 2024 and at least get back those amounts.
It's a shame Turbotax doesn't handle this properly, as I'm sure the program never asked me about a possible basis in the inherited IRA when I did the taxes for 2021 - 2024. It looks like being double-taxed on the basis in inherited IRAs could be a fairly common situation. @fanfare thanks for the detailed work-around. I'll have to decide if it's worth the effort to do that.
