Why did the NL hit first when the game is in a NL park? I'm assuming it is because the AL is the last WS winner.
No. It's because a NL park is hosting the game for the 2nd year in a row (Great American Ball Park last year, Petco this year).
You'll see the NL as visiting team again in 2018 despite being played in Washington because the next two years the game also will be played in NL parks (Marlins Park in 2017, Nationals Park in 2018).