I'm not sure the term "back forty" is all that common in Alberta or Saskatchewan - I've only ever heard that term in connection with US farms, not Canadian. But I will defer to someone from Saskatchewan who says otherwise.
I'm not sure if eh is more common east vs. west. Like I said, I know a number of people who say it, including my family, and I'm from western Canada. I haven't been to the east, so I can't really say. But if you have a/some characters who sprinkle in the occasional "eh", that wouldn't be out of place for me.
Although I should probably add that I left Canada in 2003 and haven't been back since, so my info on word usage there could be out of date - hence me hoping you find someone more current.
Side note that may be irrelevant: Calgary is also called Cowtown and Edmonton is frequently called Edmonchuck. Also, Vegreville has the world's largest pysanky, which is a Ukranian Easter egg. I learned how to do pysanky in school in rural Alberta - lots of Ukranians in those parts. There's good camping in northern Saskatchewan, like Loon Lake, which we went to on a lot of summer vacations. Lots of good camping in Northern Alberta, too.
And I certainly appreciate you trying to do it right. Very much.