Its use is similar to the upside-down question mark in Spanish, but spoken, similar to the "ka" at the end of Japanese questions.
Not really. "Jes ĉu ne?" would mean "Yes, isn't it? We might say that in English when someone has said something such as, "It's a lovely day today." However I haven't come across it in Esperanto. "Ĉu jes, ĉu ne?" I would say means "Is it yes? Is it no?"
"Ĉu" isn't only used for questions where the answer is yes or no. With "aŭ", for instance, it can introduce a question giving two alternatives. For example, "Ĉu la viro estas grava aŭ malgrava?" or that most famous Shakespeare question, "Ĉu esti aŭ ne esti?" "Ĉu" is also used to translate the English words whether and if, in sentences such as, "I wonder if it will rain tomorrow" and "He needs to know whether the story is true."