You are confusing yourself unnecessarily. As I stated, CAD can be exchanged for whatever currency you want at the FOREX.
Again, why would Canada give away free access to our market without getting anything in return? I'm all for free trade with the right countries, but it has to be two-way. Free access to our market in exchange for free access to theirs.
Let me understand you correctly. To simplify things, let's imagine two countries in the world. So Country X imposes tariffs on Canada. If Country X sells goods and services to Canada in exchange for CAD but then just sells CAD around in its country for ever, then Canada would be getting a sweet deal. We'd just have to print CAD and, in exchange for that paper, would get goods and services from that country while never needing to give anything back other than CAD that they would never use in Canada again.
As for the FOREX, why would anyone at the FOREX want to buy CAD if they can't use it to buy actual goods and services sooner or later?
Let's look at the US and Canada. Had Canada not retaliated against Trump, the CAD would just have depreciated somewhat since any US citizen who previously wanted to buy a product that Trump now tariffs would lose interest in buying CAD.
As a result, CAD would depreciate in value. This in turn would make Canadian gods and services that aren't being tariffed more attractive to US buyers. It's a little like the Dutch disease. The more a country sells, the more its currency appreciates. The less it sells, the more its currency depreciates. The market adapts.
Even if Trum raised tariffs against every Canadian product, then the CAD would so drop in value as to make Canada a very attractive tourist destination.
Add to that that it's difficult to impose tariffs at the border on software being sold online for example.
If Trump succeeded in banning US travel to Canada and cutting US access to the Canadian internet, then Americans would have no interest in buying CAD anyway. While they would no longer buy Canadian products, Canadians could not afford to buy US products either. Again, the market will adapt. A tariff is just a tax on your own consumers.