According to Hall, Shemp was "my father. He taught me comedy". I'd say they knew each other pretty well, at least when the two of them were at Universal.
According to Eddie LeRoy (Blinky in the last four Bowery movies), David Gorcey really didn't seem to mind being completely overshadowed by his brother, Huntz Hall, and whoever else had a larger part in the films. In fact, David often joked with Eddie about how they could have painted him on the scenery, and it wouldn't have made any difference. Once in awhile, David Gorcey does get some specific attention, but usually when it does happen, it involves his character being out of a majority of the film (i.e. getting injured and having to stay in a hospital bed in "In Fast Company", "Jail Busters", "Flying Wild", and "Spooks Run Wild").
And yes, the later films lose the 'gang' or 'group' feel to them. The fact that these later films have "Hail Hail, the Gang's All Here" as their theme music really doesn't help the situation.
I also missed the presence of "Sunshine Sammy" Morrison (a.k.a. 'Scruno' in the "East Side Kids" movies) in the Bowery Boys movies. I always liked 'Scruno', and felt that his character really should have been given more to do in the East Side Kids films. Billy Benedict more or less replaced him as the 'third comic in the room' (next to Gorcey and Hall). Morrison was offered a role as one of the Bowery Boys, but declined the offer, saying he "didn't like the setup". I'm assuming he either didn't want to take a backseat to Leo and Huntz (which is why Bobby and Gabe left the series), or simply didn't like the money. I think that if both Morrison and Benedict had remained with the series longer, there probably would have been more reactions in the background.