So many misconceptions, so few bullets.
1. Very few people thought the earth was flat. The obvious fact that the earth was round had been known since ancient times. People could see ships disappearing over the horizon, and they could see them come back a few weeks later, so they knew they hadn't fallen off.
Erastothenes of Cyrene calculated the circumference of the earth in 240 BC, and his calculation was remarkeably accurate.
The Earth's circumference is the most famous measurement obtained by Eratosthenes,[2] who estimated that the meridian has a length of 252,000 stadia (39,060 to 40,320 kilometres (24,270 to 25,050 mi)), with an error on the real value between −2.4% and +0.8% (assuming a value for the stadion between 155 and 160 metres (509 and 525 ft)).[2] Eratosthenes described his arc measurement technique,[15] in a book entitled On the Measure of the Earth, which has not been preserved. However, a simplified version of the method was preserved, as described by Cleomedes.[16] Modern day measurements of the actual circumference around the equator is 40,075.017 km (24,901.461 mi), and passing through the poles the circumference is 40,007.863 km (24,859.734 mi).[17]
2. Columbus completely disregarded available data on the size of the earth. He believed it was much smaller, and that he could reach India with a relatively quick trip across the Atlantic. He was completely wrong.
3. The existence of North America was known. There had been early Viking settlements in North America
as early as the 10th Century. Because they were in the far north, they grew very slowly, and in the cold temperatures of the 14th century, they were abandoned. Abandoned, but not entirely forgotten. There are correspondances among both French and English explorers who worried that they might encounter Vikings and have to surrender their new territories to Denmark.
One of the reasons Columbus sailed so far south was because he didn't want to run into the known land mass in the north. He was intending to bypass it and head straight for India. If North America wasn't so damn big and the Atlantic connected to the Pacific, Columbus might have been right to bypass it. He still would have starved to death before reaching India, though, because as noted he was completely wrong about the size of the earth and he didn't have the provisions for the Pacific crossing.
4. Canada certainly would have existed without Columbus. Portuguese sailors had seen the coast of Newfoundland in 1452, forty years before Columbus, and seasonal settlements existed there soon after, although the exact dates are hard to pin down. English and Welsh fishermen were also venturing that far west, although again the exact dates of their first landings are hard to pin down. The bulk of the evidence suggests that there were at least some Portuguese and British settlers in Newfoundland before Columbus.
From 1470 onwards, several expeditions left Bristol to search for the source of Brazilwood, which is endemic to South America. The expeditions never found it, because they were searching for it in the north instead of the south. However, the fact that they knew about the existence of this wood suggests that
some European whose identity is no longer known, had travelled to South America.
5. You're right that Columbus was not as evil as the more cruel conquistadors who followed later. Still, he was not averse to slaving and robbing the natives. The enslavement of the Taiho people on Hispaniola began on his Second voyage. Other tribes were attacked, killed, sometimes enslaved, and often robbed. And yes, his crews spread syphilis wherever they did go.
ConfederateSS.out!(The Blue and Silver Rebellion)... 
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