Thanks for taking the time to write this up and providing your complete sample application, it will be useful as we work on improvements in the future!
Regarding a “walk mode” for scenes without verticality: vtkVRInteractorStyle.h does have some api to set the movement style. The options are “FLY_STYLE” and “GROUNDED_STYLE”, I’m guessing the grounded one may behave like you want, though I haven’t tried it.
Scott