How do I recover deleted Mails on my MacBook?

I am having a problem with the Apple Mail app. Said problem is the same one in this thread; I pressed backspace on emails, and they are nowhere to be found.

I've followed all the advice in the linked thread; checked my junk mail automatic deletion settings (was already set to never delete), added all the mailboxes I could find to favourites to see if they were in there, imported the V10 directory in the library to see if they were in there, and nothing. Absolutely nothing.

I've tried a few times to delete some trash emails in the same way, and sometimes they appeared in some of the junk folders (and then later disappeared), and other times they just directly disappeared, same as the original ones.

The only thing I've yet to try is booting the Mac in safe mode to restore the OS (and therefore the mail app) in case there's a problem with it, but I'd rather not if possible given that I have no external drive to back up my data right now (I'm aware the risk is tiny, but I'd rather be safe than sorry).

Does anyone have any more advice, or things I could try? Any help would be greatly appreciated.

P.S. Topic is Older Software because I couldn't find a topic for Mail.

[Re-Titled by Moderator]