Dual types is fine with TC, done by many.
Canadians flying abroad fill in where there are not enough pilots(ME, China, etc) and will be out the day their local replacement is trained (happened before, will happened again). This is a different issue. I am one working abroad and can't get back home because TUI prefers its pilots to Canadians.
As for the training cost, that is the cost of doing business. In Sunwing's case, one ground school and 2 sim sessions would be enough to re qualify the guys every fall. It is actually chearper than 4 sims per year.
And you said that Sunwing "can't spare any", that means they are short, no?
If they would have 100 Euro pilots coming over for the winter and and 100 Canadians going over for the summer, there would be none of this. Why can't they do that?