Your chances of being killed by a tornado while on the highway are minuscule. You are more likely to be hit by another vehicle in good weather.
In a storm system capable of producing tornadoes, I'd worry more about driving through a thunderstorm. The wind and pelting rain will affect a much wider area than a tornado. In 20 years of living in the Midwest, I never saw a tornado, but did drive through heavy rain a number of times.
My nomination for most scenic would be I80 to Nebraska, diagonal into Colorado, through Rocky Mtn Nat Park and down the Colorado (more or less) to Vegas and California. But you'll be crossing part of this on the way back along I15 to Canada.
Further south, Kansas, Oklahoma and northern Texas are about as scenic as Manitoba and Sask. You'd have to drop directly south from Detroit to see much of the Deep South. I40 doesn't get scenic until the middle of New Mexico.
