I haven't seen it in a while, but from what I remember and took out of it: the whole movie is ultimately the story of a single moment in a young boy's mind, as he's forced to make a overwhelming tough choice between going with his mother or father on the train platform. The old man segments, the deaths, the marriages, etc are all fictional mental realms being generated and explored within his head as he imagines the life he would live under different choices, with the more weirder meta realms acting as sort of representations of the conflict itself and also as areas for the imaginary entities themselves to discover their own nature to a limited extent, as in the film's logic they are actually conscious entities in and of themselves.
The film was made during an era when the ideas of the multiple-worlds hypothesis and the big crunch theory were popular in mainstream cosmology, so they played heavily into the film's influence. It's essentially just these concepts represented in the framework of a child's imagination.
So overall, 1) All of these deaths were imaginary extrapolations of what he considered his future to be for each choice he could make at the station (or for a stranger take; possibly even actual alternative universes generated and experienced by the child's consciousness), even the old man was one of these, just a particularity meta and self-aware one (I suppose he was the only choice-path that managed to live that long?).
2) Acts as a both a representation of the conflict itself, and as a meta realm for the imagined characters to have limited interaction with each other and discover their nature, which allows for us the viewer to discover this with them and creates the narrative structure of the movie.
3) This one I'm less sure on, I think it's a more general expressionism thing. With the imagery of others all dressed identically to him, including the other son, I expect this was a scene that existed to largely to express his multiple existence and demonstrate to him that this place is not his world and he is a stranger in it.