I had already responded in the other thread regarding your second point.
To ensure proper the Apple Mail plugin is installed and enabled correctly, do this…
- Quit Mail.
- Quit DEVONthink 3.
- Enable Full disk access and all Automation requests for DEVONthink 3 in System Preferences > Security & Privacy > Privacy.
- In the Finder, hold the Option key and choose Go > Library.
- Go into the Mail/Bundles folder and delete the DEVONthink.mailbundle.
- Relaunch DEVONthink 3 and install the Mail plugin from DEVONthink 3 > Install Add-Ons.
- Relaunch Mail and enable the plugin via Apple Mail’s Preferences > General > Manage Plug-ins.