Blender Animation keys might be off
So you do not need to reimport the skeleton every time you want to create a new animation. You specify what skeleton you want to use, and UE4 will retarget the imported FBX animation onto the skeleton
In Blender:
re-target the anim and remove the duplicates later.
set the scale to centimeter
You can always reset the axis in the export tab, UE4 uses the classic Pitch/Roll/Yaw, Unity uses CG way(Y: vertical, Z:depth, X:horizantal)