You should be able to clip in and out with the pedals in any position.
The clip mechanisim has no idea where the pedal is.
When clipping-in, I find that you have to move your foot forward straight across the pedal as well as down. I think I tend to clip in with the pedal slight ahead of 12:00.
When clipping-out, it's the movement of your heel out that disengages the clip. Keep your foot mostly flat.
As far as I recall, there are two tensioning screws: one for one side and one for the other.