How badly will American manufacturers be hurt by retaliatory tariffs by Canada against American goods? How will all this economic warfare likely play out, in the long run?

To be honest, tariffs are the least of your worries.
You should be far more concerned about the number of Canadian citizens who, without government urging or sponsorship, have turned their backs on products from the United States. Grassroots movements have always been much more effective than governments squabbling in the so-called halls of power. Your leaders have ticked off the people, to the point that we do not mind if our boycott hurts yours.
Further, the United States has been struggling with its image for decades, being less and less reliable as a trading partner, an ally on the world stage, or as a defence partner with the world’s longest undefended shared border. As a result, Canada is finally giving up the convenience of shipping just over that border, and is finally selling (at fair market value) to other nations, many of whom the United States has also honked off.
In the short, and medium terms, the United States is going to have exactly what many have been crying for. Isolation. There will be few, if any, doors open to you. The long run, is also looking pretty bleak, with the U.S. economy in collapse, and a standard of living not seen since the days just after World War II. If you are lucky.