Lol ill explain.
Shin is Shisui, because Uchihas doesn't die that easily, they make things hard now are you telling me that Shisui actually died when he jumped off that Cliff? The answer is definitely no, he didnt die, he actually got mad when he heard that Itachi ( his chilshood and best friend ) got killed by his brother, Sasuke. This happened after the 4th ninja war just ended, thats when Shisui got back.
Shisui goes to Orochimaru to give him more power to revenge.
That is it for Shin/Shisui.
Kawaki, is the son of Sasuke and Naruto, how? Orochimaru took their DNA, Kawaki is a test subject just like Yamato.
To be honest Obito made a huge mess lol, blame Madara