Interesting questions. Here what I can contribute to them:
actionExpression and beforeUpdate seem to be new, for status the docu says:
"ID (in format of call UIComponent.findComponent()) of Request status component".
I think that relatively clear but I don't know an example for a reasonable use for that.
I tested beforeUpdate: it is called directly after the call of the action method. But what I do not understand: It expects a bean property, not a method call.
Maybe something of that is new for you - I hope the experts will answer too.