"No comingled funds."

In the case of retirement accounts, I used that term for Post-tax mixed with Pre-tax monies.

Retirement accounts are always individually owned.

"Spouse did not contribute to her IRA or do any conversions in 2024."

Does she have pre-taxed (contributions or rollovers) and/or never taxed (earnings) funds existing in any Trad IRA, SEP IRA and SIMPLE IRA? These account types are aggregated for purposes of rollover and conversion. When the answer is other than $0, any conversion will be pro rata taxable.

"She has never completed a 8606."

If she ever converted under the pro rata rules, she should have had Form 8606 and had a taxable event.

In conversion, there is no selective conversion. You don't get to put in a post-tax $5 basis and convert that same $5. Once that contributed $5 is in the account, it is considered part of the aggregated funds. When you then distribute or convert, it is taxed in proportion to all funds that were never taxed. Pro Rata.