Andrew,

Deleting a node of a polyline (OD) followed by Undo (OO) while OD is still in ChoosingNode state and then deleting another node returns a polyline where both nodes are removed.

this.polylineEntity is set once in pickEntity() altered after pickCoordinate() and a clone is returned to the drawing.
The next pickCoordinate() continues with altering this.polylineEntity without retrieving the current form and is thus unaware that the former action might be undone.

The work around to undo a removal is first to terminate (QQ) PolylineDeleteNode then Undo (OO) and then start over with selecting the polyline and indicating nodes.

While adding nodes (OA) we can simply undo the last action and continue adding nodes.

Because the new entity has the same id as the initially one ...
... It should be possible to re-retrieve the actual entity by id in pickCoordinate() or deleteNode().

Regards,
CVH