MacOS – How to close the emoji palette using the keyboard shortcut

open -a and launch seem to keep it open as well. But if you show the keyboard viewer from the input menu and close the window, the process is quit immediately. So one workaround might be to emulate clicking the menu item:

tell application "System Events" to tell process "SystemUIServer"
    tell (menu bar item 1 where description is "text input") of menu bar 1
        click
        click menu item "Show Keyboard Viewer" of menu 1
    end tell
end tell

Also, I can re-open the application after closing it via a keyboard shortcut ONLY WHEN I open the viewer via the menu button or settings.

If all windows have been closed, activate doesn't usually open a new default window. reopen would do that, but it doesn't seem to work with KeyboardViewer.

There are two types of app in OS X.
Document-based apps can have multiple windows, and they only close the window when you click the close button (red cross). The other type of app has only one window; and when you click the close button it quits the app.

Cmd+tab in OS X doesn't behave in the way you are trying to use it; it is simply for switching between open apps in a similar way to alt+tab on Windows (but not identical). It doesn't open apps that have been closed.

It's not the answer you want to hear, but I'm afraid there isn't a way to override the close button globally; it is a part of OS X's AppKit and can't easily be overridden by third-party software.

An easier fix would be not to click close when you only want to hide an app. Try pressing Cmd+H instead, or perhaps minimising the window by clicking the yellow button instead of the red one.