Should be ISceneNode::setDebugDataVisible. With scene::EDS_OFF as parameter you disable it (which should be default - so look for "setDebugDataVisible" probably you enable it at some place).
Probably distance or light-strenght. Start by adding some (not much) ambient light so you are sure to have light everywhere (scenemanager has setAmbientLight function). Also maybe experiment with different light-types (setLightData for lightscenenode), for example directional light (typically used for sun) is a little easier to set-up than point-light. You can also experiment somewhat in MaterialViewer with effects of materials+lights.