Season two is confirmed and will be based on the Last of Us 2 game, some of that game creeped into the first season of the TV show. Second season will be the last unless Naughty Dog are going to surprise us with a 3rd game, Neil Druckmann of Naughty Dog has hinted that will happen.
To answer your other questions, the TV series followed the game quite closely, in fact sometimes to the actual framing of shots, Episode 3 (Nick Offerman episode) was made up by the TV writers and Episode 7 (Ellie and Riley episode) was a DLC on the original game. They introduced the town where Joel found Tommy in this season but the town didn't really feature until the 2nd game
Casting caused a bit of controversy, many thought Bella Ramsay was wrong for Ellie but I think they were proved wrong. The origin cast from the game appeared in the show but as different characters. The actor who played Joel actually played James in E8, Ashley Johnston who played Ellie in the game played Ellies mum in E9, Marlene, play by Merle Dandridge was played by the same actor in both game and TV show. Laura Bailey who played Abby in Last of Us 2 game was one of the nurses in E9 of the TV show.
I cannot wait for the second season despite knowing how the second game goes!!