Why is Cross Game named so? What is the significance of the title?
I know jack-all about Cross Game, but I can tell you that the Japanese title (クロスゲーム kurosu geemu) is baseball lingo for "a close game" in the sense of "a game in which both teams are closely matched". Obviously, this sense of kurosu geemu is borrowed from English "close game". 

When altered to fit Japanese phonotactic constraints, both "close" /kloʊs/ and "cross" /kɹɔs/ end up as クロス kurosu. So, I guess, for whatever reason, Adachi seems to have chosen to make the English/romanized title "Cross Game".