a. All musics except battle (your route may have no battle against the monster) and Ending S ~ SSS are musics from Freesound. I found some good public domain musics. Footsteps... I'll work this out.
b. I know my English is bad. That can be one of reasons why I set game's background to medieval time. No slang to learn, characters are noble. That's why at least, I could make pure dialogues. Still it's not enough to make English based game, but I did my best. However, I'm flattered that you want to help me out.
c. You get the point. The protagonist of my game is actually, I always have been curious. What if making a game that the monsters from horror game have its own background story? And make a player control them. This initial idea made this game bigger, and even I tried to put a lesson with my terrible English skill. XD.
d. As the reason of b. I actually tried to make my game as vague. Just put a few words, then make the player calculate it. But I somehow realised if I try that, then nobody will understand my game's story. That's why I put so many monologue to the protagonist, make the player understand easier. However, I'll consider to make him quiet. Funnily, I agreed with your notion even I made this game. lol. But just thinking about protagonist's innocent. He spent his whole days by reading books and has been communicated with nobody. That kind of person, as a cliche, say too many things. But the cliche is cliche, if that make the player uncomfortable, then it needs to be changed.
e. The mapping of lonnng castle.... you may realise my intention, the bigger maps, when you reached Ending D, C. The carpet.... it's a rich castle. Well.. I need to study more about medieval castle. Not fantasy ones, real one.
f. I agree with your chase scene discussion. I'm glad fifth one makes you scary.
g. The castle has full of deception in old history. That's why, maybe the protagonist argued it before, so many locks in there. Don't worry. The castle was not a jail... or not.
Actually, you have only 2 minutes to beat the game. So it's hard to write additional notes unless you beat all endings. Because your route is the shortest route, maybe made for judges. But it's failed as I uploaded false version...
However, thanks for your detailed feedback.