My wife's contributions were also pre-tax through payroll, so it doesn't really matter who makes the contributions. I'm aware that it's not a regular withdrawal, which is why I said it would be withdrawn as "excess contributions". Thanks for the heads up!
My HSA bank (Optum) said it had no idea how to calculate the earnings attributable... I was on my own to hire an accountant. Very irritating. My best guess is that it's just the Net Income Attributable (NIA) equation, like with excess IRA contributions.
The only other question would be if I can do it at the "aggregate" level... Consider the opening balance on Jan 1, the closing balance on the day I make the correction, and the "excess contribution" as the sum of all contributions over that period, rather than calculate each one's NIA individually which seems painful
