verifying: When you send to Layout, should that imported Sketchup image always be on the “default” layer?
Yes. If the “Default” layer was the active one when the template was saved. You can and probably should make your own template(s) and make whichever layer you want active. My templates have several layers specifically for SketchUp viewports and the bottom one of those in the list is the active one. When i initially send to LayOut or when I use File>Insert, the SketchUp viewport is automatically on that lowest viewport layer.