i'm glad you liked the in game dialog! i was very worried it would interrupt the puzzle solving too much. but i will let you in on a little secret: the dialog messages have little correlation to your gameplay. most of them are based on how many times you've pushed a box around. i did figure, though, that they may be interpreted in a lot of ways as what conditions trigger them are not apparent to the player, and it's cool to see that that's true.
designing the stages was for me mostly a two-ish step process. one: draw them out on paper. this is nice as it's easy to draw, say, the intended path and think about how that will work, or to add and remove walls, etc, all very quickly. all of my puzzles were based on a core idea, e.g. "i want you to push this box over there, or through this part of the puzzle," etc. almost all of my puzzles did not actually work correctly once i built them in editor, as it's easy to overlook stuff on paper. so once i had them built in editor, i would de-cheese them, trying to at least stick somewhat close to my "core idea." sometimes i just kinda had to scrap the idea as it didn't seem doable. the other trick is to try to place walls that force a solution, but that don't make that solution completely readily obvious. this is mostly trial-and-error for me: place some walls, boot up the puzzle, and if i can easily cheese the solution, try a different set of walls.