On mine, the Automatic sensor appears to set the brightness correctly when the screen comes on, and then increases the brightness if the light level increases, but does not decrease the brightness if the light level decreases.
As far as the hinge "issue" goes, mine has magnets the hold the screen in place when it's all the way open or closed (a paperclip will stick to the back of my screen.) If I hold one half of the phone and shake it, it will fall into the half-open position, but short of that, the screen stays where it belongs.