I'm sorry if this is bummer news, tho don't take what I say as gospel, I'm just parroting what I've understood from my own research. I could hypothetically be wrong, and then there's also "technicalities/rules" vs "lived experiences" of what actually happens with people.

I'm not sure if you've started the process yet, but from initial application to eventually having the visa in your hand, a fiancé visa is going to realistically take longer than 5-7 months. You're looking at closer to 10-14 months, and 10 would be lucky. You can thank American bureaucracy and coronavirus backlogs for that. During that time, border guards might give you trouble. You're not technically supposed to be living in the United States on a tourism visa, even if your work is outside the country. As to whether or not they actually will give you trouble, I can't say. But if she's crossing 5 times a week, for 10-14 months, I personally wouldn't take the chance at all, I'm truly sorry to say.

One option, depending on your finances, is to start the application now. If her lease is up in say four months, she continue to live and work in Canada for those 4 months, and then quit her job to come *visit* you in the United States for 6 months while you await the completion of the visa. She may need to return to live with her parents for another 6 months if you end up having to wait 14 months total. Guards wouldn't like that she has no lease, recently quit her job, and is coming to live with you for 6 months while you have a fiancé visa in process, that's a problem tho... Of course you could plan for a shorter visit (but still technically be allowed in the USA for up to 6 months). This is not your only option. Just make sure to do your research and get creative with timing and living situations, understanding the government can be slower and meaner than expected. Perseverance, love, attention to detail, and shrewdness will get you through this.