In order to read the scribble on the signpost talk to the head maid in the Forgotten Village.
In order to get the bonus chapter you have to face the final boss again, and not use the jewel when prompted to. Defeat the boss and the bonus chapter will be available to you right away
You need to raise the trust level of a character you want the ending of up to a 100.
To find legendary weapons go to remote islands in both worlds, where there are no cities.