Danny's motivation
I can understand the motivation behind Danny's paradoxical character. He is alienated by European Jews because they did not fight back against the Nazis. He associates their slaughter with Abrahams obdience to G-d's command to sacrafice Issac. Furthermore, his teacher's impatience with his philosophical issues with G-d alienated him from his intellectual peers.
His character is not unbelievable. Their exists in Modern Judaism the story of how Stalin's own Rabbi had no patience for his constant questioning and threw him out of class. We all know now that embarrassing a student can have terrible consequences.
The character of Danny's girlfriend is also believable. She tells you that she was raised in Argentinia, and given her dark features it is plausible that she is also partialy Jewish. She has sex with her mother's boyfriend thus confirming Danny's belief that Jews are over-sexed.
In the beginning of the movie when he kicks the Yeshiva student he is trying to prod him into fighting back, not to turn the other cheek. He is disgusted by Jews who yield to thugs, even if the thug is G-d Himself.
The never ending staircase that leads to nowhere is Danny's vision of the World to Come. He will walk for ever upward because he believes. He believes in G-d.