I managed to beat it by using the hint book. I think it would be impossible for the vast majority of people without the hint book, or at least not something they'd have the patience for. The sound and music were great, as well as the writing, and the ending was cute. For the gameplay I have a ton of criticism but I will try to include constructive ways to improve it for feedback, because I don't want to just be putting the game down.
The biggest flaw with the gameplay was the fact that nothing interactable was visible until you moused over it. This took the pixel hunt, one of the greatest flaws of point-and-click games, and turned it into a nightmare of frustration. I understand that this is to represent Koishi's 3rd eye being sealed off, but it made the game borderline unplayable even with the hint book. I would have made everything visible, but kept the "wire mode" look of everything to represent the use of echolocation. The hitbox issues are also present on 2.5; it seemed like the problem was the hitbox being the exact shape of the object, so for objects with lots of lines it would be hard to figure out where to put the mouse so that it would stay triggered. The solution there would just be to have the hitbox be a rectangle slightly bigger than the object.
Another issue I had was with how progression works. Several times I had the idea to do something, but it wouldn't work. Then after doing something else completely unrelated, a flag would be triggered and now I could go back and do the thing I originally planned on. However, since it didn't work the first time, I assumed that what I did originally wasn't the solution. A good example is how you move Orin; I had the combined item you need pretty early on and was trying to use it on the spot you had to use it on, but it turned out you can't do that until you trigger a completely different event all the way across the map. This is the main reason I used the hint book, along with the visibility/hitbox issues. To fix this, I guess make it possible to do stuff out of order within limits. For example, as long as I have the needed items and have already tried to get Orin's cart, I think I should be allowed to do the thing that actually moves her. Having more freedom in the order you do things would have made the game a lot more intuitive for me, and I might have been able to not have to rely on the hint book so much.
The visuals were alright, though it was hard to tell what some objects were supposed to be, which compounded the issues with objects being invisible. A possible solution would be to use grayscale for object sprites, so you could have more details on them. The full color art in the ending was very cute and made getting to the ending worth it.
The audio was probably the best category; the music actually felt like the kind of music that would be in a classic adventure game. The very MIDI-sounding remix of Koishi's theme with a bit of dissonance stood out in a positive sense. The sheer amount of voiced lines was impressive. I did notice that sometimes a line would not be voiced that I'd be expect, such as a line in between two voiced lines in a Koishi monologue. Also, the audio quality on the voice lines was very inconsistent. I don't know if it was due to the VA's mic quality, or the sound files being compressed, but it sounded very crunchy and some characters I couldn't understand what they were saying without the subtitles.