Can someone point me to info about using the handles in spiro, or explain this? I'm trying to figure out how to use the handles to make predictable changes in shapes. Also - How can you add nodes to a spiro shape?

Hi.

Spiro-path is implemented by using a live path efect over a core path.
You can add new nodes with the node tool.
Select two neighboring nodes and click on the "add new nodes" icon at the toolbar.

Handle positions are disregarded, only node type -cusp/smooth- and position matters.

http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Paths-LivePathEffects-SpiroSpline.html