Question 3D Player animations by using Layers, Overrides and Constraints.

Hello evryone! If you read this in order to help me, first thanks because i'm kind of out of inspiration to solve this..
Here is my problem:
I want to make a top down shooter game in 3D this time, but i'm not really a 3D game maker so.. x)
My player have different types of weapons, he can switch between them, run without aiming, and walk while aiming. Not a lot of animations but a few issues:

• The first one is the way I've attached the weapon to the player, i just children it to the player's hand, but this way the rotation does not look good all the time.
  
  
• I'm using Layers to do walking animations while i'm aiming but when i walk the upper body moves too. And the weapon attached to the hand moves as well. For a top down shooter the weapon need to be very motionless to be accurate and i don't know how to fix this.

Thanks for attention!