Would you try to play the Hammerklavier without ever having played another piece from the classical era?
...surely not, but hey, isn't it a ridiculously impossible scenario to be up to learning the Hammerklavier without ever playing a piece from the classical era? (Unless for some strange reason all one learns on the piano right from the beginning is, say, 20th/21st century repertoire.)
Anyway, the thing is, I'm not necessarily up to playing the Hammerklavier just because I've played a few of Beethoven's other sonatas. There are challenges in said work (for example, holding the whole piece together as one work) that aren't as prevalent or just don't exist altogether in his other sonatas. It may be helpful to learn of Beethoven's other works of course, but generally it's nothing you can't achieve by familiarising yourself with his other works in terms of understanding the style. (So as regards the OP's case, I agree with everyone's suggestions regarding familiarising oneself with Chopin's other works.)
Essentially, my point is, your ability (for example, ability to present a convincing overall structure of the work) to give a satisfactory performance of the Hammerklavier (or any other big pieces) are related to your development as a pianist and musician, and not by what pieces you have learned in the past. Therefore, the question, "what pieces to learn before playing Chopin's Ballade", is not the right one to ask - and for me, shows a level of musical understanding/thinking that is not yet capable of presenting a good performance of the said work (hence my initial reply).
Perhaps expand your repertoire from merely Scarlatti to include the Liszt Sonata?
Huh? I don't get your point here.