Messages in iCloud Mail Folders disappeared when I opened the Folders.

There are two of us using the Mail program and we have Apple Mail, and several gmail accounts on the desktop and on IOS devices. I'm having a major problem only with one Apple Mail account, the other is fine ... same service provider for both accounts, same settings, only the email addresses are different.

I had several duplicated mail accounts on the iMac 3 desktop, running Sequoia 15.6.1 and Mail 16.0 sitting On My Mac, and in iCloud. Many, many mail boxes in each Mail account were duplicated. So, I decided a couple of days ago, to clean it up, remove duplicate accounts, and move our Mail Folders, with all sub-folders, to iCloud only. Unfortunately, when I was certain all messages in matching folders were the same, I deleted the On My Mac folders.

Also unfortunately, when I finished this morning, something very bad happened to one account, and unfortunately, it's not mine! Several very important sub-folders had messages in them but, as I opened them, the messages actually disappeared one-by-one from the folders.

I have six Mail Folders in ~Library and don't know which one is Mail as opposed to gmail accounts. None of the ~Library/Mail folders have a V9 or V10 file.

If I can restore individual Mail folders, I need instructions on how to do that. And if I have to, I'll restore a backup from Time Machine and return to the previous mess of duplications, if it will restore missing messages. However, I would need instructions on how to do that.

I'm in big trouble here, of course, because they are not my messages that have disappeared. That's part of the mystery. Why are mine OK, and the other user's not, when the settings are the same?

Can someone please tell me how to recover the missing messages?

Thanks.