Hi and welcome,
First of all, you having a child born in Canada and establishing domicile are two completely separate things, so you need to look at them separately.
For your child. If you have not had the baby before you both move back to the U.S. (and based on how slow Montreal is, the baby will probably be born in Canada) you need to look at the child citizenship act of 2000. Your baby will become an American citizen when they cross the border with you, but you will need to file an I-130 for them
- basically apply for their Permanent Residency - including all the paperwork, then their status changes to citizen when they enter as a Permanent resident. You will want to read this probably:[You must be registered and logged in to see this link.]Basically:What Are the Requirements of the Child Citizenship Act of 2000?
The child must meet the following requirements:
Have at least one American citizen parent by birth or naturalization;
Be under 18 years of age;
Live in the legal and physical custody of the American citizen parent; and Be admitted as an immigrant for lawful permanent residence
As for proving reestablishing domicile, I don't think you really have enough proof. You might want to look at your wife opening an American bank account, perhaps make enquiries about child care (keep emails etc that you receive), ensure the rental agreement with her Uncle is on a proper legal form, there are many you can download for free.
What about giving up your residence and jobs in Canada? When you get closer to the interview date, if you inform the person you rent from in Canada that you are moving soon, if you inform your employer, make sure you get all of that in writing and include that as well.
Also, do you have a cosponsor for the I-864?
Oh and one other thing, you don't mail the I-864 etc to Montreal, you take those to the interview. If you look at the checklist I believe they only want you to mail in the DS-230 part 1. You just mark off that you have the other things available.