That's really impressive. I wonder if an extension is capable of setting the keyboard shortcuts ? If it was then you could add an embroidery extension  to set and unset the shortcuts you use. That would help to enable the workflows simple for complex domain specific extensions like this.

Neat idea, but as far as I know, there's no good way for an extension to modify keyboard shortcuts. The only method I can think of would be to kill inkscape, modify the preferences file, and restart. That seems like a terrible user experience.

Question: - if the ice is white - how do you specify a thread change ?

The extension processes objects in their stacking order, lowest to highest. If it sees a color change from one object to the next, it automatically inserts a thread change instruction in the output file.