Robert Downey Jr. has a previous connection to the Arbus family. His father, filmmaker Robert Downey Sr. made Greaser's Palace (1972) starring Diane Arbus' ex-husband Allan, with young Robert in a small part.
Towards the end of the movie, Lionel is shown beginning to blow up the canvas raft. He later explains that it is for Diane when he takes his final swim. Someone suffering from such extremely low lung function that he will only live a few months would never be able to inflate a raft that size.
In the beginning of the movie, during the fur coat fashion show, Diane goes out onto the porch to expose herself. When she first walks out, the top two buttons of her dress are unbuttoned. The scene cuts to a close-up of Diane and the top two buttons are now buttoned.
If you were like me and jumped in halfway through the movie on HBO, for example, you are going o be utterly confused. It is likely best not to make any judgements right off the bat. I am extremely good at picking up the plots of movies, but this one you need to start from the beggining.