I still cannot reproduce this error and I assume Juergen not either and that's why it was closed. So far, your project was the only one which shows inverted horizontal/vertical constraint icons.I do not understand why this is closed, though. It is obviously not only view problem, if you want to set Z up in this plane.
A custom node class has the static method initClass(). This must be called only once and the best place where to do this is in SoFCDB.cpp -- assuming it's part of FreeCADGui. If you added it to the sketcher module then call it inside initSketcherGui.Also need someone with coin experience. I don't know where to initialise the custom node i.e. SoNode::init()