Holy toledo - 3 miles long? That's some mad engineering going on there - I pegged it at 100 stories tall ( size of tall skyscraper ) putting it at 500 meters, but that was just from impression, no math on my part ( which is still TALL ). Being 3 miles tall, that would make ascension/descension take hours perhaps, but the story reads it's traversed faster ... er, I think. Anyways, I have no clue, really, so will rely on the mathematicians here to report - and you're right, at this size, would've had to be constructed in space or on a planetoid with low gravity.
You mean, Aurang? Aurang was traumatized from his wings being clipped and had to think fast - certain death ( if he didn't open the door ) or likely death ( opening the door ). Same thing when a dirty cop has a gun to your head asking for the location of your stash, knowing if you tell him you'll likely be killed for ratting - but certain death if you don't.
I think the Dunsult didn't try to assassinate Kel ( gods tried to assassinate him, but I don't recollect The Consult trying - we never see a skinspy throw a chorae at him, for example ). I think what we see are The Consult's attacks on TGO, not Kel himself. I think they would've been able to salt Kel if they really wanted to - as we see in the final scene, with all of Kel's capabilities, just takes some chorae to finish him off. We see the hundred holding chorae only when it appears they were unsuccessful in "turning" Kel - they could've probably salted him at any time once he was in the Ark. The nuke, the scranc, et al were to thwart TGO, not Kel himself.