Thank you for the walkthrough. Before I do this, is there any downside? Am I then going to lose all of my Canadian Apple Store privileges or something silly? Sorry for my ignorance.
Also, I assume then that the Apple TV will need to always be on the US account to get all the goodies? All of my bought movies will not be there, obviously...
No real downside. With the ATV4, it remembers your various App Store logins, so you can switch between them with one click (no login/pass required).
I have apps happily living side-by-side from the US, UK, and Canadian app stores. The apps themselves don't care which store I'm currently logged into.
A few differences I've noticed:
Obviously, the App Store app will be different depending on the account you're logged into. I'm not sure how this affects auto-updating of apps. At worst, you'd have to log-in (easily as mentioned above), but it might be more smooth than that. I have my auto-update turned off for now (just so I can see when apps update), so I can't confirm the behaviour.
Siri only 'knows' about stuff in your current region, because search is done on the server. So, if your US account is logged in (and your region is set to US, which it is automatically when you log it in), if you search for movies, it will search the US Netflix catalog (and HBO, etc if you have those apps). When the Canadian account is logged in, it shows movies/TV that are in the Canadian catalog.
I get US Netflix via Unblock-US and it doesn't care what account is logged in (it always shows the US catalog), but if I want Siri to show me when a movie is available on that US Netflix catalog, I have to make sure I've got the US account logged in (one-click under accounts does it).
This causes a bit of pain, as I prefer to download apps on the Canadian app-store (for paid apps, I have to, because it's the only account with a payment method). So whenever I want to browse the Canadian App Store, I have to log the Canadian account in. Not a big deal, but not perfect.
Likely, I'll stop obsessing over the App Store for new apps, or maybe more likely, I'll stop using Siri to search the Netflix catalog. In the mean time though, it's working pretty well. If this is all I have to deal with to get HBO/PBS/Trailers content from the US, and all the terrific free content from the UK, I'll count myself lucky.