1) I would say no, since I don't see that you could save much time this way. If you would do that and people can understand what you mean, I guess, you will still get your points.
2) If the exercise ask for (and that is usually the case there) that you should use local model checking, then you should also use it. Just giving the right answer is then not enough, since that motivation for that question is to find out whether you know the rules of local model checking.