Feels like I missed a lot of content.

I'm pretty sure I'm near the end of Act 2; I gathered the three-part melody, however, I feel like I missed a lot. From occasional semi-spoilers by a random post/YT-short, I have seen certain areas and tools that I have not yet encountered. From what understand, there are no new areas in Act 3, and Act 3 is pretty short. I'm pretty thorough when exploring, however, there may be some hidden walls that I have missed...

I have two questions:

1. Are there any new areas in Act 3?
2. Can I lock myself out of Act 3 by riding the elevator up too early?