Thanks for getting back to me so quick! Deleting ngSkinLayerDisplay removed it from the main geo inputs list, but doesn't change the display. (Neither of the blend shapes showed ngSkinLayerDisplay nodes in their input list).
I ended up saving a backup and running Edit>Delete Custom Nodes. The old weight map display stays on the blend shapes, but I was able to initialize a new ngSkinTools on the main geo and new layer maps show up just fine.
The node layout from the backup file is attached. In the backup file I tried moving the ngSkinLayerDisplay upstream of the blend shapes; this didn't seem to change anything.
I then moved ngSkinLayerDisplay to right downstream of the skinCluster, just upstream of the geo_Shape. This makes the weight map reappear and work fine on the main geo!
...but the old weight map stays on the blend shape. Weird but not really getting in the way of working I guess?