Yes, you have to use your own conversion constants. The AS library uses points and doesn't expose the units choice directly, though you could probably fish it out of the plist file through System Events.

The AS library gives no access to the substructure of intersected shapes, just the UUID handle. (If you want to learn more about them you can unzip the underlying XML file inside a .graffle bundle).

And as you suggest, automated intersections thus entail using System Events to make menu choices.

Good luck !