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Go left from the location with first boss Oscar

After having enough sex, you can get a skill to initiate sex scene, each body part has one, you can look up requirements in Sensitivity Matrix

During battle, get 100% corruption on body part that your enemy is into, be horny enough (you can check by hovering over right bottom corner, status must say that sex is allowed) and hope that your enemy will activate the scene before his pleasure bar gets full or before you lose all your willpower.

You mean the interaction that you get after getting captured on the bridge? For Adventurer reputation you need to get 5+ human partners. Requierments for each title you can see when you hover mouse over "Reputation", "Partners" or "Odd Partners" in menu.

In short, get fucked by humans, literally. Just getting cum on Saria's body doesn't count

How to initiate blowjob after unlocking the "lewd technique" by getting 10 blowjobs?

No need

I have a question about the ending where MC finds the bed by themselves: did Spider kill them in sleep or not?

Spider may get me act unwise, 10/10