I think you're on the good way. [:)]

The hardest problems to solve are:
- the management of undo/redo but adapting PlanController.JoinedWall class shouldn't be so difficult,
- how should walls corner points should be computed in Wall#getPoints method when their start or end point is connected to more than one wall (by the way, you'll see that most getPoints invokers expect 4 points so I wouldn't change that if you don't want to program too much things).

Please keep us informed about your program.