Which is why Justin is suspected in the first place, but with his albi of being DED, Dresden was next in line. Elaine too would have known had Justin taught her.
I believe the Senior Council is mistaken on this point. Harry didn't even know about Simon or the Archangel site until they told Eb it had fallen. Yes, it's "conceivable" that DuMorne snuck some sort of
Sekrit Pas5wirdz for Simon's wards into Harry&Elaine's training that they didn't even realize. But I doubt it
strongly.
Elaine knows Harry's wards, and bypassed them easily because (as she explained to him) he didn't change anything from when they knew each other long before. I presume from her chiding him that she herself HAD altered her own wards, and that Harry wouldn't have so easy a time getting through hers.
But Simon? "
His tower is a fortress" Simon? Hosts the "
brute squad" Simon?
Centuries of additional experience and learning and power Simon? Can any of us believe that a senior wizard -- who is evidently of a highly security-centric outlook -- wouldn't change his wards in a frequent and unpredictable manner? I don't find it
even slightly credible that Simon's wards would still operate it the manner that Lazy-Harry or Clever-Elaine know how to get past.
I'm highly dubious that even DuMorne would have known how; it's just not good security to have that information out in the world
for decades. A whamp could seduce it out of someone, a Ramp could Turn them, a mindmage like Capiorcorpus (or Molly!) could steal it unnoticed; etc etc etc. Someone could GET that information, and launch an assault before you could learn your "trusted contact" had been compromised.
The senior council is frankly stupid and grasping at straws to think that a couple of teenagers, with an extra generation of distance, could possibly have known the
Sekrit Pas5wirdz for Simon's wards!
... I wonder if he was somehow influencing the Merlin even then to be more hasty and less considered in order to force an error. I think you are right - Peabody is no slob. I think he was being deliberate in that moment. Not magic of course, that would have been noticed, but something none the less.
I suspect Peabody was manipulating different members of the Senior Council in different ways at different times; including the Merlin.
I suspect that making the Merlin "hasty and unconsidered" would be hard; old minds tend to be less flexible, we know.
But how about making him
more considered... more cautious? How about luring the Merlin to think another layer of plan-within-a-plan would be a Good Thing, that an elaborate and subtle plot will work better than decisive action? Arrogant superiority, "we're wizards, we will win because we're smarter and think better and deeper and longer-range!" Etc...
Peabody, as secretary, would have seen all the Senior Council at work; he knows who runs easily to anger, who likes to make plans, etc. He knows their hotbuttons and triggers, what gives them pause, where they rush to judgement and where their impulse is to make allowances. He knows whose opinions carry weight on which topics.
He will look to sway the council with the lightest touch he can; if he can tip a single cautious mind a bit more-cautious, thereby gaining a majority to decide to do nothing... or an angry mind to a majority decision to act prematurely... That'll be his preference. Getting a Senior Councilmember ALONE, "please sign these forms" to predispose them for an upcoming meeting or decision, that's even better!
Every action of Peabody's will be highly situation-dependent. "Insidious" is only the beginning.