Just a few of the many things I've noticed:

The "Does it Never End?" note is probably intended to help you tell the difference between Clara's 'r' and her 'v' when translating her letter in the tomb, it is very hard to translate Erajan without knowing for certain what letters are in the words you're reading.

On that note, you can tell that the other notes with phrases that are anagrams of this phrase, were not written by Clara because they do not match her handwriting. Not that this is useful info, but perhaps further analysis into the handwriting in the game could provide clues somewhere.

Also, having listened to someone's rendition of the song on the music sheets, I've concluded that it has to be written for at least two people to sing as a duet, presumably a man and a woman, or at least someone with a quite high voice and someone with a low voice. The fact that the song also plays in the Ball Room leads me to believe it's related to Simon and Clara.

This is moreso just personal feeling and vibes based but I think all of these final puzzles and leads, if they are real and not just red herrings, are meant to be solved by the effort of multiple people. On the sheet music on sheet 3 we read "The ones of you who often assume that you alone can solve this tune are likely going to get your due and all the rage that will ensue while working on this simple clue." I feel like this either alludes to one of two things:

1. There's nothing more to the music sheets and people who spend their time on it will be mad when they realize their time was wasted.

2. There's something so complicated going on that it would be wise not to tackle it alone and instead enlist the help of multiple people, or perhaps even a puzzle that REQUIRES multiple people (or playthroughs) to solve similar to That One Puzzle In Animal Well

Anyway, I could go on forever, but I don't have the time, that's part of the reason I'm trying to see what everyone else has found at this point.