Q1: Currently the shop is not open, but likely saved for future expansions.
Q2: Could you clarify what you mean? I know that wearing the mask over a period of time will merge with your character, making them a noble, and equiping a baton accessory will make them a conductor with unique skills.
Q3: In leftmost room there's a glowing seal, interact with a crack 'n pickaxe next to it, then check either left or right hallway (depends on the character) with a gorgon braid in your inventory.