It occurred to me that you may need to meet with a very good divorce/custody attorney in Toronto to make certain, before you go, that you can somehow get trapped in Canada with her making some allegations or whatever and trying to claim "the best interests of the children" stuff once "residency" is accomplished or maintained. She should have to sign off of this NOT being a change of jurisdiction and acknowledge it's just a temporary thing; but, even that might not be enough. An experienced attorney will know the traps and pitfalls you'll need to be careful of.
For example, the Michigan custody divorce and custody might be settled but if she were to make a claim of abuse after son #3 breaks his arm under somewhat mysterious ("mysterious" only to her since she's the one looking to nail you) though completely legitimate circumstances. If the supposed alleged "abuse" occurred in Ontario, the Ontario courts would have jurisdiction and could "place" the child/children with the mother pending an investigation or resolution for their safety. I know she doesn't have much money but princess could be enabled (and financed) by her parents, family, OM and friends to bring in "hired gun" pyschologists as "expert witnesses" that will basically testify to anything and thereby help her "steal" your custody regardless of your Michigan claims to the children.
My point is, you need to be really careful and diligent about protecting your rights and those boys from your entitled wayward ex-wife. Keep a voice recording app ready to go on your phone should you ever find yourself in a face to face encounter or phone call discussion with her making threats and trying to antagonize you into doing something to her so she can get you arrested. If a child gets hurt, take him to the doctor yourself immediately and don't ask her to do it or she'll lead the doctor to conclude that you might be hurting your children and she'll try to get the doctor on board with making a accusations or reports on you. Be sure to always behave and presume to be their primary parent and rely on her during your custody time for really nothing. Even while living there, stick to the custody arrangement already set forth or she just might be able to come back to Michigan and petition for a modification herself based upon maintaining consistency.
The internet is 90% complaining and entitlement and I hate it because I deserve better!