Having the editing function depending on what is selected is more cumbersome than it needs to be.
It can be cumbersome if overdone, but at times useful.
For the most part, that sounds pretty good and consistent, but I think there would be a couple useful modifications. I'm using your outline as the base.
- If there is only one node selected, p-edit drag moves that node to the intersection with the mouse and terrain/referencable. I can't think of a case where you wouldn't want the point on top of the terrain/referencable. This would make positioning the nodes faster. This would be the only control dependent on what is selected, so it shouldn't complicate the controls too much.
- p-edit click on terrain/referenceable adds a node at that cell location. Clicking on nothing does nothing.
Regarding the addition of nodes, should this also automatically create a pathgrid object if needed?
Also, it might be problematic if the user thinks they are adding nodes to one cell and finds that they don't connect, but perhaps a note in the tooltip is all that is needed.