ENTER_FRAME (from AS3 Reference)
This event has neither a "capture phase" nor a "bubble phase", which means that event listeners must be added directly to any potential targets, whether the target is on the display list or not.
So back to your question(s):
If I add an Eventlistener on any DisplayObject it receives the Enter frame event. If I do the same with useCapture = true no Event is received.
useCapture = true // this will not do anything, as ENTER_FRAME has no a "capture phase"
But shouldn't all Events pass through the capturing phase?
Only events which have "capture phase"
If I check the Event target it returns the receiving DisplayObject as its target.
This is correct
Does the target for the EnterFrame event get changed while propagating or is a new Event created and passed to every DisplayObject?
It is sent individually to each target.
Does Flash keep a separated List with all DisplayObjects? Because the EnterFrame event is even received when the DisplayObject is not added to the Display Tree?
This is answered in Sunil D comment