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?
The actual cheese of the forge is picking the pike of returning, placing one character in the stairs, waay above, and throwing the pike into him. If it’s a fighter that’s 4 throws the first turn, each doing their respective damage + 30ish damage per throw because of the weight+height. I killed it in 2 turns, he didn’t even move since he does nothing in the first turn.