Author set up the moody dungeon making up a weird entry rule for no reason, gaining a floor for no reason, several floors suspiciously empty and as soon as the boss is down it's just "Oh well, anyway, 2 years later".
Yeah, it looked like the antidote was applied through the collar as well, so carelessly removing it could leave them without any way to get rid of the currently active poison.
You can do it, Wolca! Become Belgrieve, the Red Ogre! Well, you have the additional handicap of lacking an eye, but in manga that never comes with any problems with depth perception, maybe because mangas are by their nature always two-dimensional.
Getting distracted by random herbs and ore nodes is the most MMORPG behavior imaginable.
Seems like the assassins are former players as well ("The people of this world [...]")
So that's what the strange effect on volume 4's cover was. Then Stella will get a ghoul doppelganger as well.
First parasites, then ghouls. Both highly intelligent and physically vastly superior to humans, yet both existing in hiding. They should be the dominant lifeform on this planet.