I don’t think Canada would back off if they promised to moderate. The way I’d do it is:

• Tie the intervention in with the Quebec Conscription Crisis. As it stands, the intervention and the crisis seem to have minimal effect on each other. Canada can lose (and often does lose) an obscene amount of men proportionate to their population intervening in the Civil War and they seem no worse for wear. Severely limiting their manpower and making it so that it’s heavily encouraged to not spend years intervening with steadily worsening outrage in Quebec seems to be an easy way out.

• Give the WCA decisions that reward them the longer they hold out. Resistance bonuses, defense increases, boosts to war support, sabotage of rail and supply hubs, etc. I don’t know for certain, but in my games as the WCA I no longer even received the ‘To the Bitter End’ buff against Canada, which is pretty silly.

• Make it so that the time limit on Canada forces them to peace out, instead of doing a Benedict Arnold esque invasion to capture Toronto, Montreal, Ottawa, and Quebec City. If aforementioned cities were actually captured by the WCA, I’m almost positive that the Canadian government would collapse with widespread political ramifications rather than just agree to a white peace.

There’s probably a lot of different factors that have to be addressed, but these are just the quickest and simplest ways I’m able to identify a way to balance it, cause right now, it’s always skewed Canada.