Historically there have been plenty of incentives to stop exporting. Its called an embargo. Usually in an attempt to get the host country to change foreign policy, though I can't think of any situation where it actually worked. Examples: Napoleon's "Continental System" against Great Brtain, US oil embargo against Japan prior to WWII, Confederate States of America cotton embargo against the UK during the early years of the American Civil War

Yes it's definitely possible, but very rare and as you pointed out (especially in the confederacy's case) it usually harms the exporter much more than it helps.

I'd say that in any case of a serious Canadian export embargo, it will have been in retaliation to US trade policy or US invasion, not the other way around.

We had essentially no risk that Canada would embargo us, there was no possibility of this happening for the last 150 years until we became the aggressors