OK so I have lived in Detroit and driven to Montana and back so I know some of the route. Also helped a few friends move between US and Canada.
Regarding Customs/Border cross back over into Canada in Detroit or at the Port Huron, MI/Sarnia, On bridge. Either will be fine. In all cases of moving folks between countries the best plan is to have your daughter put together a packet showing her former residence and future residences being secured or at lease identified and most importantly some proof that she will have work where she is going. I not then you may have to fudge things and say that she is moving with her new fiance. Passports for all travelers is a must. Crossing into the US be clear on your time in country. If they know she intends to live back in Canada but that you are just taking the scenic route then they will be cool.
As far as the search goes. You have no assurance of anything making it easier for you. They may just randomly flag you for a full search and there you go. I had it once when I was travelling from work in Toronto to Detroit. They took the car apart and told me it was just how it is. Random searches in addition to the targeted or suspicious ones. If they know you are on a mission to get to your daughters new home they are most likely to just waive you on.
The western US is just like the nowhere lands of Canada. Stop for gas everytime you see a gas station. Also, in Montana they still sell some octane gas that is below what is normally Regular. So read the pump!!!.
Stop in Minneapolis/St Paul area, lots of good food and nice area. This is the beginning of somewhat regular civilization. From here on you can pick and choose your gasoline stops.
Plan a stop around here before attacking the Chicago mess. 7675 Walton St
Rockford, IL 61108. Nice small town where I used to go for work.
Hit Chicago after midnight and before 5 am. There used to be a ton of construction but now there is just a half ton. Once you pass into Indiana things should be clear.
You have to chose if you want to go into Michigan or run the toll road in OH. I always took the MI route as it was more scenic.
In MI you will have to be careful of hitting Kalamazoo at rush hour things get a little bound up around there. Then you can either charge through to Canada or stop in Ann Arbor, Mi to see the new Motor City. Ann Arbor, Mi area is the home to Toyota R&D, Mitsubishi USA automotive HQ, Hyundai North American Tech center and Nissan R&D center. The EPA has or had a facility nearby the Univ of Michigan campus in Ann Arbor which is why the auto companies moved in near by.
NOTE: I96 was under construction last I knew so from Ann Arbor you need to take 94 or go M-14 to north on 275/96 to East 696.
Now entering Detroit will be a piece of cake the economy is so depressed that rush hours are not really rush hours anymore. So you can take your time to go see the Chrysler HQ in Auburn Hills, Ford in Dearborn and GM in downtown. I don't think there is much to see at any of them, just office buildings. Henry Ford Village is great if they have events going on. Check the website before you go.
The Detroit Institute of Arts is a FANTASTIC museum of art. It is often overlooked by Detroiters same as the La Tour Eiffel in Paris. 5200 Woodward Ave, Detroit, MI 48202 Great collection of world famous artwork. If the weather is nice and it is daytime go downtown to near the GM HQ where there is a boardwalk along the Detroit river. It is right next to the tunnel to Canada so you can take a photo of Canada which is SOUTH of Detroit. There is also Belle Isle which can be nice and calm in some parts and a bit too rowdy for me in others but it is now a STATE park and Michigan State Troopers don't mess around so it may be nice everywhere now.
So if you cross into Canada from Detroit you should know your homeland from there. I have family in Toronto so I have been there many times and like it. Also been to Ottawa which is small but nice in the warm months. Montreal is very nice especially during the F1 Race.
Hope that helps.