As far as I know they're all already built in use by the CHL. The only place I can think of that doesn't have one is Kitchener, and my understand is they'll probably build a bigger better arena regardless of hockey.
local viewership is still a lot of viewership, that assuming the bigger markets are skipped altogether.
They leave all the time, they come back but they leave all the time.
You could also stagger the seasons so the c-league playoffs are completely before the NHL playoffs start.
You can't compare Canada to the US it is literally the worst comparison you can make, unless you're comparing it to college football. Which yes would be directly comparable. The league would probably be directly comparable to division 1 football in terms of regional relevance.