It depends on what you mean. If you just mean a boy who sells newspapers, then all three apply. Paperboy can apply to today's kids on bike routes. Newsboy is the general term for the kind that used to sell papers where vending machines do today, on sidewalks and corners; newsie is the slang version. (Nowadays newsie also means journalist.)
No, many girls also sold newspapers. But it wasn't as encouraged as it was for boys. People thought there were risks for girls selling on the street that just didn't exist for boys.
Yes and no. While the price hike that started the problem never was rescinded, the newspapers offered a deal that made sense to most of the newsies: any newspaper left unsold would be bought back full price by the paper.
Any grandparent must have sold newspapers well into the 20th century. Nineteenth-century newsies were street kids, vagabonds, with catchy nicknames and no place to go. The strike, in 1899, brought the old kind (Jack, etc.) in contact with the kind our grandparents would have been (David, etc.).
Spot Conlon and Racetrack Higgins existed as they were in the movie. There were newsies named Boots, Mush, and Crutchy involved with the strike too. Two strike leaders were named Kid Blink and David.
A newsie named Jack Harney was arrested during the strike. A "Jack" took part in the big Irving Hall rally. Aside from that, he appears to be completely mythical.