I don't have an NSFW problem but what do i have? A perfectionism problem. In the span of 45 minutes and 32 seconds i completed the whole gallery. Great game :)
(P.D i accidently wrote 54 minutes)
For the lava sirens (Male or female) you can either get the of the incubus or succubus first. (Or the wisps) Since they are practically the same animations which will inturn unlock lava sirens. But if you truly want to see the animation mid gameplay. For Lava sirens all you need is to have a companion, have that companion pass by the square where lava sirens reside and they will inturn be charmed (activating the animation).
As for the two skulls... Get a summoning tome (Preferably blessed), undig a grave, use the summoning tome until you get a skull and done. (Due to the skull and summoning tome skull being both ghosts the animation still counts). Hope it helps :)