Try killing your running Pantheon Mail instance and launching it with the
--debug option set:
If you're having the same issue as me, then you should see Pantheon Mail is trying to use the wrong IMAP folders for archiving/trashing mails, and this breaks the UI commands you're trying to use.
The Online Accounts dialog doesn't offer a "Gmail" provider option (even though Pantheon Mail has code for handling that explicitly) and thus the configuration for Pantheon Mail ends up being wrong.
I've found a temporary fix -- I haven't yet found a way to persist this configuration change as it is automatically regenerated.
Again, make sure Pantheon Mail is not running:
Then open the
~/.local/share/pantheon-mail/YOURACCOUNT@gmail.com/geary.ini file, change
GMAIL, and save.
Launch Pantheon Mail again and enjoy your archiving/deletion... until the next login.