On the error: Did you replace wcb.py, or did you put it in the main extensions folder? From the error, it looks like you put it in the main extensions folder, so that it can’t find the dependencies.
I’ve updated the template document with a newer one. I have tested this, and when I draw a 10 cm square, it plots as 10 cm.
> Do you know a better way to control the scale of the drawing?
Yes, try the template *without* changing the scale values.