Ah ok, yeah that’s weird… to set a bool property, you need to check it and then un-check again, otherwise it doesn’t register the FALSE assignment, that’s why the error says “property_value = nothing”, instead of nothing it should say False 
I need to fix this, thanks
Mixing animations can be done by making animations for the top half and the lower half independently, so for one animation, only key the upper body, for the other, key only the lower body, that should allow you to mix these animations together (it worked that way in the old BGE, i didn’t test but I highly suspect this still works)
As for Locomotion, I never tried myself in that before, but it would be sexy to have in there 
I’ll see what I can dig up, but I think the current 0.3.0 has problems with IK in general, as in some test I did it had a serious impact on the Rasterizer, but not in all of them, gotta check what I did there
I’d say Locomotion is not an imminent feature 
For the GUI I need to update the Add Overlay Node, but I’m having some things on my mind right now, so it could be a few days until I get around to it 