The points at which the game transition between acts seem a bit arbitrary (mainly for Act I to Act II), and I don’t see a narrative or mechanical reason to lock us out of previous maps and quests. As far as I remember, previous Baldur’s Gate games didn’t have this kind of points of no return. Why do you think they did it? Do you like it?
I’m sorry, you can hit it with the forge hammer? Hot damn.
By far the easiest way to kill it. You’ll need to lure it to the hammer spot though.