Issue 1:
Do you encounter this problem in full-screen mode?
Open System Preferences -> Mission Control. If the "Displays have separate Spaces" checkbox is unchecked. The external display will be blank, the same behavior as the other local Apple applications in macOS, say, Safari.
It works after checking the checkbox.
Issue 2:
When you find the Unity item is disabled on the main menu, suggest checking if the VMTools is still working first.
Issue 3:
I guess you clicked the green button on VM Window. That is not Maximize button, but the Enter FullScreen button as default. So the VM window goes to Full Screen mode.
In Full Screen mode, move your mouse cursor to the top edge of the screen, wait for 2 seconds, the menubar and toolbar will be shown.
If you want to exit from Full Screen mode quickly, you can press Cmd+Ctrl+F to go back to Single Window mode.
Regards,
Shan