I think slide over would work fine on the Plus 5.5" screen (sliding in from the side in portrait mode just like the iPad). They could use the iPhone 5s version of the app for the slide over app, which would leave a good 1/4 of the Plus app visible, albeit dimmed. Landscape mode might work too but that keyboard would take up so much of the screen when activated.
The 'go back' feature is good but it's not as useful as slide over. What's better about slide over is that it makes going back and forth between two apps over and over super easy and fast (while keeping some visual context of the first app). 'Go back' seems to be more of a one way thing (with no visual context).
I think PiP on the Plus should just be a split screen video that takes up the top 1/3 of the screen in portrait mode, leaving the bottom 2/3 to browse, take notes, play games, etc.