How to close a window in multi-window mode?
To answer your first question (since the rest are off topic, and the first one is your title question anyway) apps open in multi window can be shut by:


Tap anywhere in the app you want to shut.
Tap the Bar between the two windows (it may have an arrow pointing at that window you tapped, it may not)
Tap the X on the options that come up


BUT BE WARNED! If you opened an app over an app (for example, you opened YouTube, then gmail on top of it) it would switch to that app. Keep repeating the process to close any unneeded apps.