Thanks for playing, and for the feedback - I think you are right, some more polish would be great. This rating session is basically the first player testing of this game, so any feedback is highly appreciated! I'll incorporate it into the next update.
The cameras work as follows: You can see guards in any room with a camera, but when you are in a room with a camera, that camera will detect your presence as well: If you stay longer than 12 seconds in a room with a camera, the alarm will be triggered. If you destroy the camera, you won't be able to see the guards in this room any more (when you are in a different room), but you can stay there as long as no guard arrives, since there is no camera anymore which could trigger the alarm.
The lights are a somehow different mechanic - only the room where you are currently in is lit. In my original concept, I wanted to create a flashlight-like cone in front of the player, but I wasn't able to finish that one in time. (The light cone went through doors and walls, which just didn't feel right. I'll have to spend some more time on this).
Regarding the middle mouse button - yeah, you are totally right there; that will change as well with the next update. Thanks for mentioning it!
PS: Using the grappling hook to get to a floor below you is generally a good idea :)