Apple Mail: emails in an "On My Mac" mailbox disappeared
I'm using Apple Mail Version 14.0 (3654.120.0.1.13) on an iMac running Big Sur (11.6.5).
I was organizing my business receipts in Apple Mail, and I moved all the relevant 2021 messages into a new mailbox "On My Mac". The messages were all in there, but each one suddenly appeared to have no content. However, the message sizes (15kb, etc.) were all intact, so I figured the messages were there, but some glitch was keeping them from showing. I tried "rebuilding" the mailbox, because Apple recommends doing that when messages are not visible. Nothing seemed to happen--I didn't see the "rebuilding" activity notice at the bottom left. However, shortly after, all the messages except for those from one vendor (which I had done a Spotlight search for at about the same time) disappeared from the mailbox.
I'm thinking now that the "rebuilding" discarded the messages, and somehow the Spotlight search preserved the ones from the single vendor. I didn't understand before that rebuilding meant the local messages would be thrown away. But I don't think the messages are on my email server anymore, since I previously moved them into an "On My Mac" mailbox, so I can't re-download them. (Apple should make it that you can't hit "rebuild" on a local mailbox.)
So my question is:
1) If messages in a local "On My Mac" mailbox are deleted, do they go into some trashcan before disappearing permanently? I looked, but couldn't find them.
2) Could I get these messages back through Time Machine? This post indicates no, but I thought that you could do that before. (I wish Apple had told me that my email was no longer being archived.)
3) Is there any other way I could get these messages back? It's for my taxes.
Thanks to anyone who has ideas.
iMac 27″, macOS 11.6