Here are some ideas how the editor might achieve that. As the user draws such a polygon from scratch, a closed outline is maintained at all times, by rubber-banding the last boundary segment as well as the closing boundary segment to the first vertex. It is prohibited to draw self-intersecting contours. Probably all offending contours involved in the intersection should be highlighted in some form, giving the user the option of adjusting the previously designed contours rather than the one most recently moved. In this mode, the cursor snaps preferentially to vertices, then to edges.
Illegal situations stay highlighted in some form until they are corrected.
Regions can have additional informations attached to them that is not always explicitly visible. This extra information should be readily retrievable with a simple mouse or menu action. Many of these issues might preferentially be checked in a batch mode at times when the designers has reached a certain stable point in the design activity.