Frankly I never use GY520 be4. So I don't know the very cause of your problem now. Did u test the servo with other gyro? Just in case?.

1. Delay is response time from gyro to the servo. Normally for digital servo; delay is 0 coz the servo itself is already fast enough to catch instruction from the gyro.

2. Limit function is crucial. It's create the gap that servo only travel on certain length before it's bind.

3. you can subtrim the rudder, but one time only. once u have done the limit and setup the tail then leave it like that. If u have do some adjustment on gyro after u crash; u need to redo it back again. But it not recommended.

4. The very step is set the slider at the center before do the limit function. limit function only set left portion will be the same as right portion (limit).

5. Nope. Bind only happen if the servo limit is exceed the length of right/left side of tail slider.