This looks good, but it's not clear that it would actually work in this scenario. The "Recovery Security" section of the first document is good, but it requires knowledge of some combination of the phone password, an "iCloud security code", possession of the SMS number for the user, etc.
The latter two links are good, but both only refer to recovering access to an iCloud account, they don't talk about further access to escrowed iCloud keychains.
Recovery is handled differently depending on who you are using to sync your passkeys. Apple has a great recovery story. iCloud Keychain supports both a "recovery key" and "recovery contacts".
See the section titled "Recovery security" in the "About the security of passkeys" doc: https://support.apple.com/en-us/HT213305
Additional relevant docs:
Recovery contact: https://support.apple.com/en-us/HT212513
Recovery key: https://support.apple.com/en-us/HT208072