Facial twitch: I believe these twitches are similar to the twitches people get in their eyelids. Usually it goes away within 6 weeks or so. There are other causes such as caffeine, stress, alcohol, electrolyte imbalances and excessive fatigue. Feel better.
A shutter speed of 1/30 for example will let in twice as much light as a speed of 1/60 since it is two times slower. When you decrease the shutter speed by a factor of two you are increasing the amount of light by one stop. Remember that fractions get larger as the number on the bottom gets smaller.
But if you increase the focal length of your lens to 400mm, you must also increase the shutter speed to 1/500th of a second to ensure a clear photo. Depending on the amount of available light, you may not be able to use a fast shutter speed like 1/500. This is where image stabilized lenses come in.