Right- so post clarification attempt.
I've tested TheatreMode and the inbuilt visualisers and they all work as expected and function great.
Thanks so much for your hard work, it's fantastic!
For people who would like to use the Milkdrop plugin, there seems to be some strange behaviour related to fullscreen, and Milkdrop just being touchy as.
Currently Milkdrop when put into fullscreen on the secondary monitor, it grabs focus and will not let you manipulate the primary window with MusicBee in it.
In order to fix this you need to enable Milkdrop's inbuilt 'fake fullscreen' mode through the ini. This mode creates a borderless window rather than switching to an actual fullscreen mode. This is needed as a single display adapter will treat the second monitor as an extension of your first and thus you can't have fullscreen in place in one but not the other.
If you add the following line to your milk2.ini in C:\Program Files (x86)\MusicBee\Plugins\plugins (or wherever you have installed) it sorts it out great.
I'm still getting some strange behaviour with window controls, hotkeys, and closing it... But nothing I think I can adequately describe.
I've used the default milk2.ini and one generated and customised by Winamp to troubleshoot various things. Seems the fake_fullscreen is the only required to get things mostly working. I think Milkdrop is going to be a bit janky no matter what.