For the other thing, you just have to select the second option where it says there will be more content with the woman in the forest, then talk to the cat boy, and then go at night to where the woman is in that cave in the forest. An option will appear allowing you to see what they are doing, and then you'll have it in the gallery. 

I already know how to defeat the secret boss that appears after defeating all the other bosses. To fight the secret boss, you have to climb the trees, specifically where the frog girls are. After defeating him, he drops an item that allows you to fight the slime queen. At her door, click on the item to access round 2. After defeating her, you'll get a cutscene, although there's no animation yet.