I'm trying to do my first top down assy. I'd like to insert existing sketch geometry to the layout, but SWX keeps telling me "The item on the clipboard cannot be pasted here." How do I copy existing geometry to a Layout?
No idea. It might be a limitation caused by the fact that your sketch entities are coming from a 2D Sketch, while the Layout is a 3D Sketch... But that not explains why the block workflow works.
Are those entities part of a 2D sketch? The workaround is to make them a block first. Then you can copy & paste the block.
Just tested this approach and it works.