I think this whole saga might just end in nothing. Before these swingeing taxes are actually imposed, the US International Trade Commission (a separate organisation from the Department of Commerce) has to rule that
imports of aircraft from Canada threaten material injury to the domestic industry
http://enforcement.trade.gov/downloa...lim-092617.pdf
Since Boeing chose not to offer Delta a viable, Boeing-built alternative to the C-Series C100 aircraft, I fail to see how any "material injury" has been suffered.