So assuming someone else owns the game/can better understand Japanese, help an idiot out here: this is all in the version that just recently released, right? I just finished downloading it and a bunch of my old saves, but I have no clue where the heck this new stuff is located. I'm currently just running into all of the areas and seeing if anything new pops up, but no luck so far. Somehow I'm not surprised that my attempt at picking up an RPG in another language that I haven't played in practically a year is failing spectacularly.
I'm starting to think that my super old saves are incompatible, though; I just noticed that the heroine is missing her textures for her skin on her overworld sprite when I use them, and there are no spots for new scenes available in the recollection menu (assuming they were actually added there). Crap.
Update: so I found the orgy scene after paying more attention to the picture on the blog (and downloading the newest save I could find in this thread) - it's next to the bar where you can have the heroine dance. Interestingly enough, the succubus is dark-skinned and has blonde hair for me. I haven't been able to find out if I can change her appearance yet, but simply leaving the town and reentering didn't affect it. Also, that position you see on the blog, with her straddling the heroine, is pretty much the only one that involves her; the rest are just a collection of all of the old positions, with the heroine by herself (though it's nice being able to access them all in one place).
Still no luck on finding that dungeon with the egg-laying yet. I also found out that the missing textures might not have been due to my old save, but the heroine's skin tone; I changed it to a darker shade, like I had in my original save, and it happened again: