If I remember correctly though, the ink was meant more for the Senior Council. As they said, the older the wizard, the harder it is to get them to bend. Though it has more to do with the mind being settled in the body, vs the actual age of said mind, hence why Lucio could be manipulated. The ink was there to help him push and nudge the senior council into doing what he wanted.
Plus, remember, the young ones were baptised in fire. They got plenty of education on the hot and heavy evocation side, but they probably skimped out on the mind manipulation portion of the curriculum. And they were just so dang young. They probably had no clue what was going on, since they'd never experienced it before. Most of them were anywhere from 16 to what, 25, maybe 30 for the oldest like Carlos? I'm 24 and I'm still experiencing all sorts of new stuff.
Never underestimate inexperience.
I don't but at the same time relatively young Harry recognized when Corpsetaker was in his head back in Dead Beat. His defenses sucked because the Council frowns on anything having to do with mind magic including defenses against the likes of Corpsetaker, but my point is, he
knewshe was in his head. We all assume that Peabody used some kind of mind magic in addition to the ink.
Perhaps, what I am saying the ink not only made the mind vulnerable, it made it impossible for the mind to perceive that anyone was in it. Or Peabody didn't need to get into anyone's head, the ink did all the damage and all he had to do is suggest. All that was needed was a drug, administered by
the ink that entered the bloodstream via the skin and was designed to target the part of the brain that deals with will power and judgement. It was also subtle in how it worked because the effects
were so slight that no one noticed that Luccio or Eb or anyone else on the Council was acting different. The effects were cumulative and affected all that came in contact, so it was simply not noticed. It would have continued except for the fact that Harry had never came in contact and was determined to clear Morgan. Had Peabody succeeded in getting Harry to sign papers like he was supposed to do, Peabody would have won.