Just a note on dates: we don't have much evidence to go on the 1142 date. Barbara Mann and Jerry Fields are mainly the people behind that date. Most scholarship backs the c. 1451 dating. It's based on a number of assumptions on how people in North America would respond to solar eclipses (the main hint we have from oral history for dating the formation). It requires a lot of faith in my opinion. It's certainly not impossible, but considering how amorphous and short-lived most political organizations in history tend to be, the more recent formation date is easier to swallow in my opinion.
Regarding Cahokia, while that particular city is thought to have been abandoned during the 14th century, the Mississippi valley civilization that created it is thought to have continued well into the 16th century, so your question is still valid. Hernando de Soto's expedition up the Mississippi documents many towns and cities that are thought to have belonged to it.
As for significant interaction or trade between that civilization and the northeast Iroquoian groups, my guess is it would have been very limited by geography, distance, and rival factions. Even at their height, the Iroquoian population density would have been dwarfed by the Mississippian city states. It would be like comparing upstate New York with New York City. So I doubt the Mississippians would have much interested in trading with them beyond establishing tributary political relations.
One reason for this is that we have good reason to believe that the epidemics that ravaged Mexico and the Mississippi in the 16th century didn't hit the Northeast until the 17th century. This suggests limited trade. It's possible that people in that area didn't have much reason to exchange goods and news, or maybe the distance was just too vast absent the horse, wheel, or sail.