1) in the Animation menu set click on Skin then Detach skin.

2) You have set the weight of influence on each joint per vertice, you can do this either with the paint weights tool (found under skin/edit smooth skin ) and paint 0 influence on the verts that you dont want to effect. Search Modifying skin weights in the help docs.
Another method would be to use the Component Editor where you can directly modify the values of individual skin point weights. Search Editing skin point weights in the help docs.

3) You might solve with when your weights are painted but if not then you will most likely have to add some more resolution in those ares so there is more vertices to influence.

4)If i understand the question you are talking about setting Keyframes and or auto keying the animation ?
This can be found in the help files by searching Keyframe Animation.

Hope this helps a little for you .