As far as I know, there is no overlapping communication when it comes to communication with air traffic control. Each plane has their call sign by which it is addressed, in Frankfurt/Main there are different frequencies for the north and south runway and I believe, planes are told the frequency on which to communicate. If there is a lot of traffic, one air traffic controller is responsible for one runway, another for another runway and a third is responsible for air plane ground traffic between terminal and runway. (There is a German TV series about how Frankfurt/Main operates which is incredibly interesting and fascinating (if that kind of thing interests you, of course)).
If there were overlapping communication then it is the pilot's responsibility to double check. A pilot cannot, under any circumstances, do anything because he believes that this was said.
That goes for the second point as well. A pilot can never believe or assume anything that wasn't specifically stated. It's double check and triple check when it comes to flying because when accidents happen, they tend to be catastrophic. So, if the pilot did, indeed do anything based on assumption or believe then it would be on the pilot.