MacOS – How to change keyboard shortcut for opening Character Viewer on OS X El Capitan

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 doesn't seem to be any way to insert it using the default U.S. input source or other normal keyboard layouts:

$ grep -r '␣' /Volumes/Ukelele\ 2.2.4/System\ Keyboards/
$

Unicode Hex Input is the only keylayout that uses multipliers. If you use Unicode Hex Input, you can insert ␣ by typing 2423 while holding option.

Or create ~/Library/KeyBindings/ and save a property list like this as DefaultKeyBinding.dict:

{
    "~S" = (insertText:, "␣"); // option-shift-s
}

You could also make a new keyboard layout or add characters to the press and hold popovers. I'm currently using an AppleScript (posted in this question) to insert special characters and plain text snippets.