Okay, so, regarding H-scenes and how to interact with them. I like to document these things for some reason, so here goes.
There are 4 enemy girls and 5 monsters. Also, you.
The Zombie and Ratdog can rape an enemy girl solo or in pairs, with the order in which they jump on the girl affecting what plays. Zombie then Zombie, Ratdog then Ratdog, Zombie then Ratdog, Ratdog then Zombie. Applies to all 4 girls. Gotta catch 'em all.
Unfortunately, animations can be rather buggy. When the Ratdog attempts to DP, they sometimes start humping at what looks like discarded clothes and not the girl herself, like the animation is supposed to.
The Plant and Orc-thing have one animation for each girl and cannot pair up with any other monster.
The Insect is unique in that most of its animations are unfinished, so it borrows otherwise unused/rare animations instead when raping.
Other enemies cannot join in if an Insect reaches a girl first.
But, if a Zombie or Ratdog is solo on an enemy, *then* the Insect can jump in, but not the other way around. This usually changes the animation to something crazy, but sometimes there are actually appropriate animations.
The monsters can also fuck you, the player, and can be provoked when masturbating (V key), when SP gauge is full via getting fucked/gathering lots of hearts/MP, or in special rooms where the monsters are automatically hostile.
Gangbang rules that go for the 4 enemy girls also go for the player. Animations with the Protagonist + Zombie/Ratdog + Insect work properly with rather appropriate animations.
Speaking of the Insect, if you have full SP gauge (arousal meter) then the Insect summon will one-shot enemies. They play a little 'stinging' animation when doing so, and certain girls will get bit in the tit. So if that turns you on, try it out.
Also, there are Background NPCs and rarer Animated CG scenes. All Background NPCs can either have a Zombie attached to them (and the zombie removable with close-range attack) or cannot be interacted with. There are too many Background NPCs to count, but the Animated CG scenes are limited, so I'll document how to reach them all, just because.
The first animated CG is on 1F. There's a locked door in a room found in a tiny hallway, which can be unlocked by defeating the lone shotgun girl. Once opened, you find a girl that's all tuckered-out from being raped a whole goddamn lot. If you press A to check on her, zombies rush into the room. You can destroy them and prevent the girl and/or yourself from being raped, but if they reach you, you get a special animated CG scene.
On 2F, go up the stairs and then go to the right. The second door in the hallway there leads to an empty bed. Rest on that bed and you'll get an animated CG where a zombie titfucks the sleeping player character.
3F has two animated CGs. The first is found in a hallway full of ceiling-bound plants that spit cum at you if you're under them. Enter the door in the center of this hallway and you'll enter a room full of insects and a girl that's been all raped-out. Press A on her and you'll be ambushed and raped by insects. For this animated CG, pressing the Cum button actually just removes the insect that does you anally, so it feels a little lackluster.
There's a plant in 3F stuck in a tube. IIRC it's on the way to the final room, so it's impossible to miss if you know the path to beating 3F. To break it open, you can't rely on assistance from enemy soldiers since there are none in the room. You need to spam zombies and then destroy them with your attack, causing them to explode and damage the glass. Might need full arousal to give them the explode-on-death ability? One cheat ability causes monsters to explode more violently than usual, which is what I was using when I figured out how to break it, so that helps.
When you beat the game you get a 'gallery' mode that's accessible from the Start menu. You can purchase elements with hearts, then drag them around the place with left mouse. (There are still a few tiny little animations that you can't see here, like the girl-on-wall-of-tentacles-but-without-cum-on-her animation and the girl-being-raped-in-tube-of-tentacles-while-upright-in-background-of-1F-hidden-behind-pane-of-glass-and-obscured-by-the-damn-wall animation, which I THINK is separate from the other tube-of-tentacles animation. The dev must be forgetful or lazy.)
I think that concludes all the important H-scenes that you can find in the game, besides finding every Background NPC. If I've missed anything, feel free to say so.