For others....
I spoke to an IRS agent and they said this: Box 7 of the 1099-R should indeed have been code "P." And no double taxation should occur so long as the excess was distributed before April 15, 2023 (and it was indeed taxable on 2022 return). I should ask the 401k provider to send a corrected 1099-R. If they don't, I should contact the IRS again and they will write a letter to the 401k provider asking, pretty-please, to do so. (I assume they'd do it at that point, but if not, the next steps are unknown to me at this time.)
