Perhaps the Plane problem should be fixed by using Quaternions under the hood. One good reason for that - if I understood it correctly - is that an Indentity Quaternion with its four scalar values can be thought of as a “vector” plus a scalar for the rotation angle (which is reset to “no rotation”, which still is a defined rotation on the unit sphere) would take care of the ill-defined Planes. The missing rotation of a naked vector in space that is.
I noticed that Unity and ogre3D and other engines use Quaternions under the hood, and Eulers only in the UI, since Euler rotations is considered more intuitive by most developers.
In any case, the Plane problem is a PITA, especially GH is dragged down in the dirt becaiuse of it. Quaternions perhaps is the pill that cures the ill.
// Rolf