All the scenes are unlocked via a certain way, getting 100% is optional. The hidden mode requires inputting a code the main menu, it's easy to guess the code, but the mode itself is not easy.
Yes, it's possible, though you have to know the game inside and out practically. Sasha's mode is harder because you can't access Seiryu Village in the demo, but that's temporary. For the first boss you have to be sure to go in with around 20 bullets, and always line up the pistol crosshair perfectly, because it does almost twice as much damage that way.