Cool, but technically, I thought the two background split view apps are suspended when using slide over... but I could be wrong. Test with apps that have video/animation running?
Interesting. The background no apps look greyed out like you cannot interact with them. Is that the case? If so, might be that way when split screen is in background? Seemed from the keynote, you could still interact with a single background window app.
Not possible on Air 2. You can only have split view and the video pop-up. You can also have one app running in fullscreen and another app running in that hovering window while watching a video in the pop-up player and you can interact with them all.