You could create a custom BehaviorBase-derived class whose StartDraw method returns one of the following depending on where the mouse cursor is located:

// to move a node;
return new InteractionState(node, Action.Modify, 8);

// to create a link
link = CreateLink(originNode, point);
return new InteractionState(link, Action.Create, link.ControlPoints.Count - 1);

// to draw selection rectangle
return new InteractionState(Diagram.Selection, Action.Create, -1, point);

We'll try to implement multiple resize in the next few days.

how do I know if the cursor is on the drawing area on a link or on one node?

And on a AnchorPattern?