Were the St. Lawrence Iroquoians Iroquois?

No. They were two distinct Amerindian nations which belonged to the same great linguistic family, the Iroquoian. In the era of Cartier's voyages, the Iroquois inhabited northern New York State, whereas the Hurons, who were also Iroquoians, lived in the area surrounding Georgian Bay in Ontario. As for the Amerindians known by the name of St. Lawrence Iroquoians, they occupied the entire St. Lawrence Valley, from Kingston to Tadoussac. In the period separating the attempt at colonization by Cartier and Roberval in 1541-1542 and the arrival of Champlain in 1603, they had completely dispersed and no longer inhabited this territory.

How long did it take Jacques Cartier to cross the Atlantic?

During the 16th century, crossing the Atlantic lasted an average of 50 days, starting from Saint-Malo, in France. It is believed that during his first voyage to Canada in 1532, Cartier probably benefited from particularly favourable winds, as he managed to cross the Atlantic in only 20 days. However, in 1535, the same crossing took 50 days. As for his third voyage in 1541, Cartier required a full two months to cover the same distance.

How do we know that Jacques Cartier wintered on the location of the Cartier-Brébeuf National Historic Site of Canada in 1535-1536?

Jacques Cartier's narratives of his voyage tell us so. In them, he mentions that he found a harbour at the junction of the Saint-Charles and Lairet Rivers, which he considered an ideal location for sheltering his ships from the winds and tides along the St. Lawrence River. This haven is where he wintered with his crew in 1535-1536, on the very location of the Cartier-Brébeuf National Historic Site of Canada.