Although I don't have answers to all of your questions, our situation wasn't too different from yours. I'm a Canadian citizen and my wife is a US citizen, we have 3 children and have been married 7 years. We were living in the US and a job in Montreal fell into my lap and we decided to take it and move up here. The plan your lawyer suggested is the same one that we took.
As soon as I accepted the position (late-October 2019) I applied for citizenship certificates for our children (online), choosing the option and writing an explanatory letter with the application that we intended to move to Canada by January 2020 and the children would need the certificate for health cards, school, etc. We received their certificates by mid-December. One thing that might help: you do NOT need a Canadian passport to travel into Canada for a child as long as you have their citizenship certificate and a US passport. The child would come in as a citizen and not with a passport stamp or other visa.
We applied outland in December (see my signature for timeline) and I came in January to get a house ready and then flew back in February to get my family and fly back with them. Upon arrival in Canada we explained it exactly as you wrote it: I took a job here, we've applied for PR, however we recognize that my wife is a visitor and will return to the US in the event her status expires at any time. They checked over the childrens' citizenship certificates and then gave my wife a visitor stamp for 6 months. There was really no issue or questions - the border agent said that a US citizen applying for PR as a spouse is more a formality than anything else. In retrospect I wish I had asked if we could get a visitor record for, say, a year, for her to avoid having to apply for the extension. She applied for a visitor extension in June because the stamp expired in August, but we haven't heard anything back on that yet. We're still waiting for the PR process to move forward as well, COVID-19 and the special Quebec process having slowed it all down.
I provided documents about my intent to move to Canada but it was more concrete for me. That being said you just have to prove your intent to move and it sounds like you can do that. I showed evidence of the accepted job offer, quotes from moving companies, etc. Notwithstanding where you finally settle, I think the forms just ask if you plan to live in Quebec or elsewhere in Canada, and another question about where you plan to live. I think you could enter anything here so long as it is at least one of your plans. I'd suggest perhaps near family, or a large city with the jobs, etc.
Best of luck with the application and the move when you make it.
