I like this a lot, I look forward to playing the completed game :) On the whole it is very good, and I can see a lot of effort has gone into it.
I have some comments and suggestions, contains mild spoilers for those who haven't played the demo yet. The first three are things I think that are important to change, the others are minor nitpicky things that you may want to look at. These are all meant as constructive criticism so I mean no offense by any of them :) Just want to help.
Spoiler
1 ) Maybe the walk speed could be a little faster?
2 ) Although the translation is pretty good, I would suggest getting someone to go through the dialogue again and make it more readable in English, as the subtle differences kind of detract from the atmosphere. Also the title screen hasn't been translated, and when I found the book on the floor in the library, the page that came out wasn't translated so I had no idea how to proceed from there.
3 ) It would be useful to be able to press the up and down arrow keys on the keyboard to cycle the inventory up and down, especially as you only see three of the items at a time. I think manually cycling them by clicking could get tiresome.
4 ) While you are looking at the 'loose page', if you then right click on the recipe book the game crashes with an error code (Error: DisplayMessage: Invalid message number to display).
5 ) If you take the recipe book down to the room with the broken bike and then examine the book, it says the book is a saddle of a bike.
6 ) When you move the cursor over the blank piece of paper in your inventory, it doesn't say anything in the bottom left of the screen as it does with the other objects.
7 ) Engel says "here I go" when going up the stairs, this makes sense the first time, but if you go back down and then up again, I don't think he should say it again.
8 ) Once you've already looked under the carpet and got the key, when you look again I think he should say "there's nothing else under there" rather than saying he's found the key again.
9 ) Once you've used the key to unlock the upstairs door, or used the wedge to open the other one, I think the doors should stay open so you shouldn't have to use the items again each time you go through them.
Good luck with the rest of the project.
You're very welcome, glad you found it helpful :) Sounds like you have a good plan to make this game really fun.