1) Associativity can be taken from the integers, since we know the integers are associative, a subset of them will be as well. Inverses we do not automatically obtain, but it is obvious that if g = 2k in 2Z, for k in Z, then -g = -2k is also in 2Z.  But, we need to explicitly state that and verify that it's the case.

Closure is done correctly. Good job!

2) Correct.

95/100, since you didn't explicitly state how the inverse of an even integer is also even.