Brennan apparently finds Em within minutes of landing in New York. You ever played hide-and-seek in New York?
Chances are he had already found out where she was living before he arrived.
He probably called her Dad to find out. We never found out what he thought of Brennan, he may have liked him.
Amusement parks stay open late. Kennywood for example, Adventureland's stand-in in the film, closes at 11pm. So where do Brennan and Lisa P. find a fancy restaurant open that late and is that crowded in Pittsburgh?
It was probably their day off.
Em calls out Sue for anti-semitism, asking her if she hates gay people and supports apartheid also. Good for Em, but the film was set in 1987 and homophobia was, sadly, quite common, even among people who might otherwise consider themselves progressive-minded and in any event would be in no way considered comparable to apartheid.
Actually, that's false. It would be the same. Apartheid was a racist institution. It would not be unfathomable to see otherwise progressive-minded people support racist institutions like apartheid because it's common. It's passive acceptance. One may not say or do racist things; however, if they stand by and do nothing, they accept it.
And there's no reason why Em couldn't personaly be ok with gay people. It was '87, not the 50s.