🇺🇸United States @sam452

Looking over the stacktrace of this error appears to invoke the theme's use of wrappers around the node entity. Without that theme this procedure normally works. 

Thx @wombatbuddy as that is the right answer.
That gets one thing done and digging into the linked Form wiki I can see that I would need to refer back to the #ajax selector in a second addCommand() like so:

$response->addCommand(new OpenDialogCommand($selector, $title, $content, $dialog_options, $options));
$response->addCommand(new ReplaceCommand('#myselector', $form['allowed']));

return $respnse;

The ReplaceCommand acts as the original callback to the $form object (inside the callback function).