[HerenIstarion]

In the books, as opposed to the movie version, Aragorn has got Narsil's shards as early as his first appearence in Bree. In Rivendell, the swords is reforged and renamed. The event is even an indirect cause for Bilbo to give Frodo the Sting - Bilbo intending to have Frodo's broken sword reforged as well but forgetting about it.

So, Aragorn does have Andúril on him in Fangorn

The reason why Gandalf could not be hurt is probably (without textual evidence, but as plausible a theory as I could think of) the fact he is now 'clothed' maia, returned to his original form (though maybe spirit returned to his 'human' body lying on the summit of Redhorn), not fully and finally incarnate as he has been before. So even Andúril would not hurt him - how physical thing can harm spirit?

Why Andúril (or Narsil at its time) does work with Sauron (plus the Ring) is also explainable - Sauron is just recovering from his second loss of bodily form - what he rebuilt for himself is the 'true' flesh - he is incarnate, not able to 'shift' bodies anymore. If we stretch the thing a bit too far, in that state he may have been killed with a fork