Basic question about Animation blend

Hello,

I have a plan to build my own locomotion, and I am wondering something about animation blend. As you know in order to experiment animation is has ready to use animation, before I reached this step. I have something to ask first:

1. How does animation blend's work?
I want to make good precision turning animation, I've tried making walking animation blending with stand animation but it's like the character's lean to left or right. What I guess is character can turn diagonal because rotating are basically standing but tiled instead of straight. but What I got isn't like that, it's more like transition between these two animation. Maybe it's seems I have answer my own question but I just to make sure because I want good precision turning movement, so how many animations do I have to make to represent each degree of rotation? As I remember, starter pack asset has 0, 45, 75, 90, 135 and 180 and multiply by two because we need to make counter direction too (left and right)

2.What is Animation mirror?
This is related on first question. in humanoid avatar, we can see that animation can be mirrored. when I make 45 degrees turning left, if I toggle mirror option, does this mean animation will changed to turning right?
I need to mirroring turning but sometimes I need to mirroring without changing their direction, for example:

original = 45 degrees turning left with steps: Left foot, Right foot, Left foot.
mirrored = 45 degrees turning left with steps: Right foot, Left foot, Right foot.

Because sometimes walking into turning isn't always start at same foot.

as far as my observations, every animation executed concurrently but it's depending on which animation is more prominent.

3. is extra bones worked in human avatar?
I want to add twist hand bone, see this Rig arm twist - maya. the bones appeared between "human avatar" bone. Does this not interfere with the avatar?