The Fallout for this particular creature is a little different than typical. Under normal circumstances you have to take Stress damage to your CTRL stat to take Fallout, but in this case it's passing like a pathogen through physical damage. So any time the Investigator takes physical damage from the Dark One, they're at risk of both a Wound and Fallout instead of one or the other. The chance of Fallout triggering is a bit tipped in the players' favor, but that's because taking Fallout here can mean having your character ripped from your control much earlier than they otherwise would be under normal circumstances, so there needed to be a bit of a balancing act so that you don't have a player left twiddling their thumbs 10 minutes into the session.
As for the behavior of the Dark One, that was a bit limited by space. But the implication from the Tactics section is that they're going to try and escape the room and then try and infect the rest of the floor. NPCs don't need to follow the same mechanics on spreading Fallout as the Investigators, so you can just convert them if the Dark One is reasonably able to get their hands on them. The Dark Ones should then be differentiated from each other by taking on aspects of the personality and behaviors of those they've infected.