Simple answer - no.
All you are doing is using the Canada STB the same as if you were sitting in front of it in Canada.
So if you changed the channels at home with someone alongside, youl're changing the channels away from home.
Same thing same result.
So two solutions - get the Pro-HD tuner and watch that which leaves your set top box alone for those in Canada to enjoy without you changing channels.
Buy a second dedicated tuner for your use only connected to the component inputs on your Slingbox.
Yes it applies to all Slingboxes that use the Set Top Box at the other end of the link. Change the channel on your Slingbox feed and you change the Set Top Box at home.
I run a dedicated Slingbox Tivo so I can watch, record and playback anything without interrupting the 2nd Tivo in the main lounge area where others can still watch that Tivo without intteruption from me.