They do not need permanent settlements for it! After they have crossed the Niagara portage to Grass Island Pool and Grand Island and built sailing ships for Upper Great Lakes, they can sail all across Erie, Huron and Michigan, stop briefly for trade in each Iroquois or Ojibwa village , sheltered bay and rivermouth... and then sail back or onwards.
And St. Mary Rapids are relatively nearby. What the Norse need to settle is the portage - to new ships built to sail Lake Superior.
Why Manhattan Island of all East Coast islands?
If the Detroit-site isn't required as a link in the network of Norse markets, maybe the Iroquois take advantage of the site, and if they get to the point of replicating Norse vessels, they could try to exact a toll on ships coming and going. A situation like this may lead to future conflicts.
The Norse need to try and get on with every autonomous tribal-nation in the region. Be it intermarriage between chieftainly dynasties, or setting up a market just for the benefit of individual chiefdoms. These subsequent emporia founded after the really vital portage stations would be the foundations of later Metis states.
Maybe just any island inside the Hudson river where OTL New York city is located. Perhaps Ellis Island might be defensible? To take advantage of the naturally-suited harbour.