I have quite a bit of Joplin under my belt and have studied ragtime for skool, so I'll try to answer those questions as best I can.
1) Joplin did not "invent" ragtime, merely popularized it. At the time he was writing, ragtime had already been established (particularly in St. Louis, where many publishing houses were) by other composers such as Tom Turpin, Joe Jordan, Artie Matthews, Louis Chauvin, and others. In fact, romantic composers had already been experimenting with the ragtime rhythms! Chopin's Op. 25, No. 9 etude seems to me to have many of the qualities of a ragtime piece, minus the l.h. octaves... Brahms and Debussy wrote some pieces almost like ragtime, although they didn't gain popularity then. In 1919, after the heyday of ragtime, Stravinsky wrote his Piano Rag Music. But undoubtedly, Joplin was the self-proclaimed "King of Ragtime Writers," and the one who got it off the ground...
2) Certainly the roots of jazz exist in ragtime, which was largely a "black" music spawned from slave songs. Jelly Roll Morton, whom I consider to be a ragtime composer because of the clear l.h. meter and traveling octaves, called himself the "inventor of jazz." His music had the distinct rhythm, form and chord progression of ragtime. Without ragtime, there may never have been jazz, and everything that developed from that point onward, including blues, rock, r&b, et al.
3) Ragtime has a distinct chord progression that repeats itself throughout. (e.g., The Entertainer has a repetitious C, C7, F, C, G7, C, G7, C C7, F, D7, G7, C, C7...etc, etc, etc.) Then there are the aforementioned l.h. traveling octaves. Also the syncopation is a factor. The form is similar to sonatas: the first melody is repeated once, then travels to a variation (however distant), which is repeated, and then loops to the original melody. When you think that's the end, the composer adds another theme that repeats, followed by another that repeats...then the end.
4) I'm not sure what the grade is, but they might be acheivable if you have the hand span to grip all the octaves. As an added bonus, you can take it slow, and it will sound better than if sped through. So I find it easy to sight-read through most ragtime pieces. With practice, it is definitely possible!
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