How does his second effect work? Lets say my opponent declares an attack to my Maxwell with his 4 drop at 7 attack. I say its legal, then he activates his Hala to give his chararacter + 3 attack. Can I then discard a location to Maxwell in response to Hala, and make his 4 drop lose the Kree affiliation, thus he would not get the attack bonus and still be at 7 attack?

You can do that, sure---target legality is always checked at both announcement and resolution.

You can respond to anything with him unless some other card says so.