It isnt, the detector isnt either but the detector only shines in ng+ and you need the detector for the exp acc to shine. Its by the river in the forest, just examine every tile to find it. I believe its left and up a few tiles each from the bridge but memory hazy atm..
Left (maybe both?) Alt is the auto toggle. If you alt tab a lot then you might have accidentally turned it on.
Basically the slums unlocks via the various slums related Mercy routes. I couldn't tell you the exact ones but the White Dog is part of the "Destroyed by a giant" ending (fuck the bandit and choose not to live with him) but doesn't really have any major scenes. You get one picture from memory of her being fucked by the giant but nothing else to the best of my knowledge.
You need to level up a bit more. There are 3 sex skills that are unlocked by levels but I can't remember at which levels.
You get a skill per dog fought (I think?) up to the max of three, which feels really awkward design wise, since it means you have to go back/use the erotica once you can finally do it. Plus, there's more than 3 dogs, so...
Sewer Devil there's one for sure. You need to save him by waiting before attacking him during the quest and to trigger the ending, talk to the thug while naked and fuck him and follow him to the slums. Decline to stay with him and during the first day there fuck the 3 random npcs (not exactly sure the requirements but that's what I did) then talk to the boss so you can go to sleep. The following day, head to the store talk to the boss and exit the store when given the choice. You should get the choice to trigger that ending then.
It's the door to the right, yeah. If nothing's happening from trying to enter that door after running away from hoodlum (either choice after following him and then just leaving) then it's probably something to do with the various triggers there are before becoming FEMC. You need to have met Vampire in North Castle, presumably not killed him, and I'm not sure if you can do this after killing/sparing the sorcerers.
Not sure if you need NG+ or not to access it in the map where you save Merchant, the one with the cliffside to the north and forest to the south, there will be areas fogged up to the north. The 2nd one I believe will have a hidden pathway if you walk past the fog that leads to the Dream Pavilion. As for the usage of it no there doesn't really seem to be usage. If you got the Mercy zombie village ending, she shows up in the place with Evals and the dog and you can talk to them as well as sit in one of the chairs by the table but it doesn't trigger any event or anything. Red Sorcerer can also show up (no clue on the requirements) and gives some lore behind devils that I'm not sure if it's 100% unique to that part or was mentioned somewhere else in the game.
