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?
If you do the underdark, then port out and do the mountain pass, you can also be just about level 7 before A2 which makes some of the early fights there quite easy
Found this out on accident when I realized I’d skipped the pass for the second time and trudged back, determined to finish it.
Still haven’t seen these harpies people are talking about tho. Playthrough 3 I suppose.
Explore the druid camp more if you want to find the harpies.
they are lvl 3, in the coast near the grove, inside the grove.