Yeah, once Ebenezar had seen the evidence on Peabody, responsibility for how to play it kind of is on the SC member rather than Harry.
Politically, they couldn't really have the Merlin just go "hey, trial on Morgan is cancelled" anyway. The spectators from LaFortier's bloc had to be convinced that Peabody was responsible because of how much suspicion was going around that Langtry would subvert the process to rescue his favourite attack dog. It wasn't just Harry that failed to anticipate the "bureaucromancer" posing a physical threat - Ebenezar and Carlos also missed that.
Right, and Ebenezer should have walked up to the Merlin, said "Hey, we need to incapacitate Peabody with thorn manacles before the trial—trust me, man, this is going to be
awesome." I mean, the guy might seem like a little nobody—the Butters of the White Council (pre-Ghost Story, before he leveled up)—but he's still a wizard of the White Council. There's some level of competence and power required. Why
wouldn't they have anticipated him running? It was in their best interest to restrain him, but they failed to do so completely. And, you know, fifty wizards died while Harry played Poirot.
Actually, Harry's summation speech let Peabody prepare; he realized where this was going, and pulled out another inkwell during Harry's testimony, which was the one with the mistfiend in it. Or... some kind of mist that summoned a fiend somehow. Did he bottle a mistfiend, actually? Because that's pretty cool. Anyway, Harry should've made sure the Merlin was ready; there was absolutely no reason not to tell the whole Senior Council, so they could have all been prepared adequately. Privately, the blame lies mostly on Ebenezer's shoulders—he was supposed to be ready to shut him down, and wasn't—but publicly, it was Harry's grandstanding that got them all killed. Which they then used to force their guy onto the Senior Council.