hmmm.... Thought long and hard, I have. A difficult yet easy decision at oncem this was. And so speak normal I shall.
I believe that Nat would have had the capapbility to summon an Afrit and restrain him without tiring very much, if at all. John, thanks to his determination and skill, was the strongest at that table. Simple common sense. While Whitewell is older, meaning
possible higher experience level, John's stamina and experience in hostile situations up to that point would have won out. These include the Lovelace Conspiracy; The Golem incident; Case of the Afrit, the Envelope, and the Ambassodor's Wife; the affair of the curiously heavy trunk; and the episode of the Anarchist and the Oyster.
Addmitedly, we don't know anything of Whitewell's Backgroud or experience, but it's still clear that John's been through much more, practically, if not Politically. Whitewell is known to keep at least one Afrit, Shubbit, who favors taking a bear form. The strength of this entity is unknown, but if was able to survive the encounter with Nouda, protecting Whitewell with a formidable shield, until she was killed, and he set free. I find it likely that she had at least one, probably two, Djinn for other tasks.
Her defiance was legendary, and no one on the Council dared defy her, something not spared to Nat, who was openly hated among the coucil. Perhaps not too openly; that risks upsetting the Prime Minister.
Nathaniel would have summoned his Four Greater Djinn, Ascobol, Cormocodran, Mwamba, and Hodge, and Bartimaeus for personal protection, using the others to attack. This is assuming he did not know the attack was coming.
Whitewell, on the other hand, would have summoned, oh, lets be optimistic and say her Afrit and three Djinn to defend herself. Nat would have sent Mwamba and Cormocodran, his two most powerful, after Shubbit, and have Ascobol and Hodge stay with Bartimaeus near him to fend off and kill Whitewell's three Djinn.
Assuming neither get the chance to run, I say that Shubbit kills Corocodran with a mighty Detonation, only to be felled by succesive detonations afterwords from Mwamba, then Pierced by a Silver-tipped spear Mwamba found on the wall.
Bartimaeus would kill one while narrowly avoiding the others, giving commands to Hodge and Ascobol all the while still trying to get Nat away.
Eventually, Ascobol and Hodge are seriously enough injured that Nat send them back, even though Mwamba comes back in time to kill another, leaving the four of them to finish off the weaker one Djinn left.
Nat turns, sees Whitewell leaving, gives an order-
And a particuallarily Power Joint-Detonation from Mwamba and Bartimaeus erase dear Jessica from the face of the planet.
Knowledge is Power, Power Corrupts, and Corruption Destroys.
Life is pleasant. Death is peaceful. It's the transition that's troublesome.
~Isaac Asimov
There was never a good war, or a bad peace.
~Benjamin Franklin
You can kill a man but you can't kill an idea.
~Medgar Evers
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