jfitter, thanks for the explaination! But this is IMHO an explaination of the inverse yaw. As you say, in inverted flight differentials act against a good roll (if I understood correctly your post...did I?

But the contraddiction is that in pattern airplanes we need differentials and they help ALSO in inverted flight rolls.
I'm pretty sure this is true: in my pattern plane, if I remove differentials rolling capabilities are bad even for rolls starting from inverted flight!