Bugzilla

I just did a copy of all my bugzilla folder to Local Folders and they seem fine in Local Folders. I haven't check with imap log or wireshark, but a "move" from imap folder to local folder should just mark the messages in the imap folder as deleted. So if you set the delete mode in server setting to "just mark as deleted" you should still see the "moved" messages with red X and lines through them. Then you can undelete them. Note: if server is gmail by default any message marked \deleted is expunged so you won't see it with red X and line; however, it might still be in All Mail folder.

I just did a copy of all my bugzilla folder to Local Folders and they seem fine in Local Folders. I haven't check with imap log or wireshark, but a "move" from imap folder to local folder should just mark the messages in the imap folder as deleted. So if you set the delete mode in server setting to "just mark as deleted" you should still see the "moved" messages with red X and lines through them. Then you can undelete them. Note: if server is gmail by default any message marked \deleted is expunged so you won't see it with red X and line; however, it might still be in All Mail folder.

Edit: I just did a "move" (instead of copy) of multiple messages to local and I don't see a problem. 
>Further, perhaps as a safety measure, if messages are not moved properly there should be some way of retrieving the originals?

When I set the imap account to "Just mark as deleted" in server setting and look at the folder, the moved messages do have the red X and line through them and can be undeleted. So this acts as a sort of "safety measure". Then you can return to "move to trash" delete method.