Hello again! And thanks for trying out our entry. Proud to be the first you were willing to fight through to the end (and that you enjoyed the ending)!
I appreciate the list of pain points! Some are interesting to hear, like the vent areas and the closed window in the library. I had intended the vents to simply be dark maze areas, the first with a mild penalty for missing the jump (you'd have a longer way up the dining room), and the second just being a straight maze. Was there anything in particular that made you want to explore them instead? The closed window (in my mind it was a door) in the library was me just trying to make it make some sense of the layout of the house. I put the door there to "make sense", but it wasn't where I wanted the player to be, so I closed it off. I'll be more careful with the visuals for something like that in the future.
I hadn't thought of making a forced "death", but it's a good fix for some of the design problems I'd run into (and easy to explain premise-wise with the blossom spirit doing teleports already). Trying to make different parts of the room contain different parts of the main path (i.e., the dining room has path 1 through the middle, path 2 along the bottom, and path 3 along the top) was tricky for the checkpointing. I think it suffers most there and in the dash part of the top of the library. (We did provide a path from the bottom of the dining room straight up to the top that was possible once you had all the unlocks, but you're not the only one who felt like they had to go back the long way, so I wonder if there was a better way for us to point it out.)
Thanks again for the feedback and the nice praise. Glad it wasn't too frustrating in the end ; )