Beautiful work :)

I still have not used IK in blender 2.40, but the main problem is that direct IK chain from head to parent bone, when that IK chain moves force to parent bone to rotate. I don't know if that's becouse of that Automatic IK option, but I couldn't break it either changing hierarchy.
I think the best is build again the skeleton, this time put the parent in the hips. Divide spine in two chains, from hips to shoulders and from shoulders to head. Parent those chains, legs to hips and arms to shoulders.
Also make a little arc with legs/arms according their movement, to force automatic IK to run in the right way (I think it will work).