The U.S. car insurance won't be sufficient unless there's a specific rider on the policy. They need to check with their insurance company. Their driver's licenses will only be valid in Canada for 6 months after landing - whether they're there or not. So if they're not coming back in time to get Canadian licenses, they'll have to find another way to get into Canada. Just FYI.
I'm not an expert on car importation, but here's what I read from
www.riv.caWhat to do at the border ...
Produce your title documents, registration and sales receipts.
Complete Vehicle Import Form – Form 1, which Canada Border Services Agency will provide. This requires a Canadian mailing address. I stopped there because of the last sentence. This tells me the vehicle importation process will not be completed at port of entry, but later on. So I'm going to say it will be ok if they bring their car in on landing and return to the U.S.
Just to make sure, give the Canada Border Services Agency a call. They'll know the answer. (506) 636-5064