Scenes that made no sense to me, please explain.

I've seen this movie several times and there are still some scenes I never really understood the point of and which I'm still curious about. The first scene that made no sense to me was the opening scene where we see Solomon in the Nazi clothes looking like he's drowning and trying to save himself before we see somebody saving him. What was that scene all about? And what did that have anything to do with the rest of the movie?

There was also another weird scene after the Polish guy was run over. We see Stalin and Hitler dancing with each other in the orphan which was already destroyed and we see the Polish guy almost like a statue bleeding on his stomach. Was the Polish guy killed or what? What did that have to do with anything else from the movie? Was it some kind of dream that Solomon had when he fainted because he was a communist and then a nazi so he in some way made Hitler and Stalin friends with each other?

The last one I didn't understand was the one when Leni talked about food and about her fantasy and then covered her head and asked if Solomon is still there as if she wants to make a joke and make him laugh but what was that joke about? Why didn't he react to it and why did she get so mad at him when he tried to kiss her goodbye and she pushed his head away?