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?
Well, all Jaheera and Isobel told you to go to the towers to investigate how to disable Ketheric’s invulnerability and that you could pass through as a True Soul thanks to your condition, so… yeah? You had reasons to assume that it would be a social zone.
The act 1 to 2 popup doesn’t say that previous zones will become unnaccesible and this does. That’s a pretty big indicator to me.
By the way, I think that it’s possible to go down below through the prison and reach the final zone before dealing with the nighsong, I have to try it in my yet next playthrough.
The level indicator thing would be nice, but this won’t happen with Larian, since they don’t treat quests like quests you are expected to do, but as “stories” that you write up in your journal. The issue here is that effectively they are quests and that you are expected to do them since they give EXP and you need it not to get stomped anyway. Here I agree with you but don’t expect changes.