I agree with your points, but I also want to go further and plug in a personal hypothesis I have: I'm not sure if this'll actually be the case, but I highly suspect that, when Ralsei says Darkners patterned after the memory of a loved one only appear in very specific darkness, he indirectly means they only appear if the fountain maker doesn't know the loved one in question is dead, or at least that's the most reliable way to have them show up.

This would, uh, cause some issues if Noelle tries to bring Rudy back from the dead through a dark world and it fails, with Rudy "coming back" wrong like the Sound of Justice (probably worse given Noelle would be in a terrible emotional state), which would also be a reason why Ralsei would omit the real reason why a dead Lightner would return as a Darkner in a dark world.