To query the actual layer visibility, call AcMapLayer.IsVisible(). There is no method to set actual visibility because it depends on other visibility settings.
To query the visibility setting for a layer, call AcMapLayer.GetVisible(). To change the visibility setting for a layer, call AcMapLayer.SetVisible().
To query the visibility setting for a layer group, call AcMapLayerGroup.GetVisible(). To change the visibility setting for a layer group, use AcMapLayerGroup.SetVisible().
To change the layer visibility for a given view scale, modify the layer resource and save it back to the repository.