In the first season, the door next to Val's door is proven useless. Her apartment is too widely spaced for that door to lead to a neighboring apartment, and it's entrance would have been covered up by a large wooden dresser.
What happens to Ben? It seems that they just kick him off after Holly is done with him
Gary says he is away for two months yet in the fourth season he does not come back
How come Gary and Vince still live with Tina?
Gary told Vince that they had an exterminator in the apartment because he let people stay in it while he thought Vince was going to be away so he stayed at Tina's with Gary. However months have gone by since then and they still live there probably so they are right across the hall from Holly's apartment.