You're conflating domestically produced GHG emissions with domestically consumed products' emissions. As a toy example, suppose we grew no animal products in th

You're conflating domestically produced GHG emissions with domestically consumed products' emissions. As a toy example, suppose we grew no animal products in the US and imported all our meat. Your method would conclude that switching to plant-based diets would have no effect on GHG emissions since livestock represents 0% any of our agricultural GHG emissions.