Fantastic demo and very satisfying puzzles, especially the last one! I was stuck for a few minutes, but then I had the "a ha!" moment that keeps me coming back to puzzle games over and over. I saw your response to another comment, and like that person said, I think you should keep it like it is, no hints needed after a certain number of tries.
One bit of feedback: the intro dialogue is a bit of an exposition dump, and you get a lot of the same information from the letter itself. Perhaps try a shorter opening dialogue, "can't believe I'm coming back after all these years..." etc. but then leave the stuff that is a bit redundant within the letter itself. Maybe after reading the letter, have the character comment on how strange it was that none of their uncle's friends admitted to sending the letter. Just my two cents! While I play a lot of point and click adventure games (I was immediately intrigued by your inspiration of Rusty Lake and Submachine, favorites of mine), by no means does my opinion match what other people will think, so please take it with a grain of salt.
I'll say too that the atmosphere and music hits the right spot, and I actually got a little creeped out and decided to close the shutter behind me to stay a bit safer, lol. Great work and I look forward to the full release!