Disagree (England brought up in North West, live in South now). At the bar is standing waiting to be served in most cases. "In" implies in the building. At would be more normal for at "name of bar" but "in the pub" sounds more normal to me than "at the pub" unless they were in the garden.
Hmmm. I don't see any comments by "Maria", but, according to http://aztekium.pl/przypadki.py?szukaj=bar&lang=en& , it's genitive.