I think some of the enigmas serve the story and the time. Admittedly, my "time" is a couple of decades later but Even then the bar-centric life was not unusual. I had quite a few friends I met at bars (whether platonically or otherwise) and some of them are still my friends, but there is not always that much of a back story. Our stories are more going forward: who we were then and what we became etc. So the idea that we need more exposition never occurred to me. (I'd also note that these guys are definitely seasoned drinkers, in a way that develops in bars.)
Re Michael running, I think it creates a really fine and classic enigmatic ending. I want to think he is running back to Donald before he leaves, but he could be having another panic attack, or he could be going to kill himself. We don't know. (I mean we kind do because of the other play but that's cheating.)
The apartment is supposed to be on East 65th and looks like it could be but again there are indications it could not be. The theatre district seems present in some of the late scenes including Harold and Cowboy in the taxi and whatever is in the distance from Michael. I don't know how much it would have cost in the 60s. In the 80s it might've been under a grand I think. Faded elegance for sure. Today (pre-pandemic) I think you have it about right in that condition, but I would think $2mil would not have been enough to buy. I'm sure someone will have more of an opinion.