As far as the vacancy issue goes unless you do a very wide area they'll keep moving back in. I've been dealing with gsquirrels for most of my life. One of the best control methods is deep plowing right after they summer hibernate. I hire a neighbor with a 600hp Big Bud set up with deep shank duck feet. It's actually for subsoiling but it get the gophers good. Breaks through the hardpan.
My dad used 1080 when it was legal, but I know it kills off target and wouldn't even consider it. One bite and they would run back down and die. Sometimes they wouldn't make it back down and the predators would eat the left overs...not good. 1080 is super strong nasty stuff. I think it's still available in Canada
An uncle used cyanide crystals, worked pretty good but a jar of that would be impossible to get today. A small spoonfull in the hole and cover with dirt. The moisture in the soil activated the cyanide.. Oh yea my misspent childhood..
ED