I like the symbolism between Harvery and Donna to Mike and Rachel. They have been doing it all season, with all the different interactions showing that if they don't act on their feelings they will end up in the same situation as Harvey and Donna. Then in the past two episodes they show Harvey losing his dad, who was his main influence in life, with Donna being the one to break the new to him to now Mike losing his grandmother, also his main influence in life, with Rachel being the one to break the news to him.
I knew something was going to happen that would leave Mike having the appartment for himself. The way they played the Mike and Rachel looking at the apartment hinted at it.
It was probably only the Summer last year because it was the first season. USA typically does that to do a "trial run" before greenlighting it for a longer season. After seeing the sucess it had I knew it would follow the Summer and Winter routine they do.