I took out travel insurance because I won't have health cover. Even my wife has to wait 3 months before Ontario will offer her the state cover. My decision had nothing to do with immigration.
I went with an Asda policy which covers you for up to 90 days per visit. I'm back at xmas so this should cover me in two chunks up until March hopefully. It was their more expensive policy - the basic one was 30 days and not N America or something, so read the small print.
With CBSA I'd say just be cool. It's a nervous time because so much rests on them letting you in, but it's a computer touch screen you're interacting with mostly. I was sent to a desk because I put 90 days as stay length. If you have a return ticket for 3 weeks and put 21 days into the computer I'd bet you wouldn't even have any interaction with CBSA, you'd be waved through.
What passport does your partner hold?
She has got an EU passport. When it comes to the return ticket we would most likely book a return in 6 months.