Arcade is in the sewer. Once you get it it'll appear in the upgrade tree.
The game over screens are all unlocked because I haven't implemented any kind of roll back function so there is no way to unlock the game over scenes in the gallery. To get the Alexa ending start by feeding the cat 3 times. Then put the camera on it in the sewer.
Otherwise there is a lot of content in the sewer that is kind of optional and you don't find just by rising corruption.