The double light is telling you the gyro detects a rudder input from your radio. It happens a lot during operation. When the rudder stick returns to neutral position on the transmitter in AVCS mode, the light will go solid on again - provided that the gyro was last reset with the stick in this position. As you advance throttle, you're likely adding a small amount of inadvertent rudder and that will be enough to trigger the light signal on the gyro. It is normal while in AVCS mode.

When you say the nose rotates to the right, when is this happening? You're not using any Tx trim on the rudder are you? Does the tail drift in one direction while in normal mode with no Tx rudder trim? In case you don't already know, tail drift in normal mode should be minimized with linkage adjustments with Tx trim set to neutral. Don't use Tx trim on the rudder to compensate for mechanical issues! Drift issues should always be first addressed in normal mode. Finally, do you carry the model while in AVCS mode? If so, you should reset the gyro's neutral before engaging the clutch by quickly moving the rudder stick back and forth three times and then letting it snap back to neutral. Otherwise, it will immediately attempt to return the model to boot-up heading.

100% gain is most likely too much for that gyro/servo combo and excessive gain will overwork the servo. Because the 401 apparently doesn't hunt on some helis (on mine it does) when the gain is too high, use just enough gain to get the job done for your level and style of flying. 70% is a good starting point in my opinion. If, one day, you find the tail isn't holding during fast backwards in HH or the piroette speed is inconsistent during quick-traveling piros, try bumping up the gain a bit.

Hope all this helps.