General questions about IK/FK and animations workflow

I'm using Blender, and deciding what I want my workflow to be like between Unity and Blender. Right now there are a few questions;

1. Should I make animations in blender, or in Unity? What are the benefits of doing it either way?
2. What is the standard method for switching between IK/FK in the middle of an animation (maintaining positions), while ensuring that it works alongside things like IK'ing the feet to the ground dynamically?
3. What issues arise when it comes to fading between animations dynamically if you do animations in Unity VS in Blender?