Heya!
So happy that youre interested in the game still and id be glad to help ya!
For the writings on the grave, they are dates. Each grave represents someone in the manfred family, to solve the writings we count the Y-axis of the dots and move to the right on the X-axis to get the next number. For example, if the dot is 2 dots up our first number will be 2, then we move to the next dot and so on!
The chess board on top of the lever is used to solve the maze, the king piece on the chest moves on a specific pattern that follows the "salt circles" in the maze. If we move on the path that the king piece moves, and we count the sides of the circles (which are actually n-gons), we will get a series of numbers. The numbers are a1z26 which means that each number represents the position of the letter in the alphabet.
The third floor is known as the attic, it reveals two things. One is the lore implications of both adam and ulrich, and the second is a cipher in that room. We know that each even in the game is tied to a letter on the keyboard, and by looking at the vat of randomly changing objects and slowing it down we can tie each object in the vat to an event (which in turn ties to a key).
All of this is in the Crow 64 Game doc so if you want any help or want to revise on anything then that doc is a HUGE source of help!