1. How does the game know how many players are playing and which animals they're using?
When the game is started, the electronic voice asks the players to push the buttons of the animals which are playing. After that, the players push the "?".
2. The rules mention three difficulty levels and a switch under the compass. I can't see any switches from any pictures, does the compass lift up?
Yes, you lift it for finding a switch and the battery storage. The switch has 4 positions. "0" for "off", and the numbers from 1 to 3 for the different levels.
3. The helper animals are mentioned as only being able to help the players once per game. Are there other animals or are they referring to the toad/millipede/bat, etc etc?
Nope, only these. So you better ask an animal not already asked, because you will not get a new hint.
Also, if I understand it correctly, Hubi appears as soon as enough magic doors have been opened (a player on either side of the door, one player tries to move through the door), and then he starts to move around the house. Hubi:
* Can move orthogonally and diagonally
* Can move through any kind of wall
* Cannot move into a space with a player (again, how does the game know where we are?)
* Cannot move outside of the game area
Plus: Only to adjacent rooms (incl. diagonally)
The game always "knows", where everybody is, because the starting corners are fix and the programm "knows" the labyrinth and everything ...
I presume that on harder difficulties Hubi can move more than one space?
Not as far as I know.
Edit: One more question. The game mentions that you have to catch Hubi before midnight. Does the game have some sort of internal timer?
Only on 2nd and 3rd level, there is a time pressure implemented. This is not working on "real time", but on the number of moves you need to catch Hubi. Additionally, the hints of the animals are less precise and helpful.