Should be able to save apps to full screen mode whenever they are launched DeX mode.
When you exit the application, it should not save in your multi app folder and should be closed completely until reopened.
YouTube does not operate the same as it should on a pc (clearly). When using Youtube whilst searching on Google Chrome, it opens the app which is not capable of full screen multitasking.
The way Samsung makes it easier to use without you having to Pay for the Premium on YouTube, is by having either YouTube open, and having Docs, or Microsoft Word open in a smaller window. So you can have apps open while working on homework and you are able to quick tab from work to your games.