What I always heard is that it's because oranges used to be seasonal: until the invention of reefers (refrigerated boxcars, not marijuana), they could only be shipped from Florida to the population centers up north without spoiling during the winter.
Isn't the red Christmas outfit something Coca Cola came up with? So only since the 20th century? I think I've read the outfit was mostly green before that.
No, but it's a myth so common basically everyone believes it to be true.
The Santa we have today is an amalgamation of a few characters. The red outfit comes from Saint Nicolas. You can just look up "Saint Nicolas" and you'll find many fairly old depictions of a Christian bishop-like character in red garments carrying a bag of gifts.