The only way I know of that Inventor can export to excel is with the Export BOM tool once you finish making the BOM in the idw. You can use the tool I have highlighted in the image to bring up the column chooser tool which will let you choose exactly what columns to display. As for exporting a header per assembly, there is no tool inventor has that I'm aware of. There isn't a temp. style that can really work with it either. I'm sorry I couldn't be of too much more help.