I don't know, the prologue had Lady Lancaster predicting

Strife such as has not been seen since the breaking of the world.

I think we can be pretty sure there was some sort of cataclysm, but if it was a terraforming accident on one planet of a multi-world empire the Builders were from, why not just evacuate to another world rather than eke out an existence under siege? Plus, the reference to horses as 'fantastical animals the supposedly existed long ago' strikes me as more suggestive that this is a wrecked Earth.

I'd actually wondered whether this might be some sort of far future post-apocalyptic Dresdenverse. Maybe the price that magic users pay has mutated from tech disruption to madness. The surface creatures could, potentially, be Fomor guard beasts that got out of control and infested the planet.

All I can say is the references to Archangels have me waiting for Mr. Sunshine to make a crossover appearance.

Cavendish was busy memorizing the Index, and there was the possibility of two etherealists countering whatever she might have done so I can understand why she stayed out of things.  Barring those circumstances, I can see how the scale of power from the crystals might be a limiting factor.

I wonder whether it might be an issue of range as well as cannon crystals being too powerful to counter. The only things we actually have examples of the etherialists doing at any range longer than across a large room were Folly's tracking beacon and her interception of the Enemy's communications - all the Kaboom! effects we've seen so far were relatively short-range.

I might try to get a question about this in next time Jim does an AMA.

That phrase about death being lighter than duty caught my attention too, but I think it's a homage rather than a subtext that the Spires are supposed to be an age in Jordan's mythos.  The eccentricities of the etherialists pale in comparison to WoT's madmen going on city-buster scale killing sprees until they're brought down or rot to death from the taint. Even Cavendish was at least metastable compared to that.

Plus, a male having a female apprentice wouldn't have worked in Jordan's universe, where each uses different power sources.

I'd wondered whether the fact that he had that up his sleeve when he threw the hilt of Fidelacchius to his friends contributed to the blade's transformation.

The one problem here is that once Nic was helpless, Michael would have started talking to him again, trying to save him.  Harry would have had to do the ice spear trick then run over and grab the noose.  Or the frigidus-forzare combo and then picked up the noose from the Nic chunks.

I think Michael was committed to ending him by that point.  He's smart enough to realize that ice wouldn't hold Nic forever once Anduriel set his attention to freeing him.

Considering his 'emotional attachment' to it, I would be surprised if he passed it off at all, or at least not so quickly. Once passed off, there is a good chance it would eventually wind up in the church or buried under some Wizard's sub-basement.
There are 30 coins, plenty for Nick to choose from.

But Nick *doesn't* have the others to choose from for elevating new henchmen.  Tessa and Rosanna made off with the bag containing most of the loose ones at the end of Small Favour, so Nic had to resort to retrieving Ursiel and Lasciel to rebuild his cadre from Death Masks.  Unless he went to extraordinary lengths to rescue them in the few minutes the escape from Hades took, those two coins are lost to him again for the long term.  Even if Saluriel (Cassius' Fallen) and most of the others in Church custody get busted out soon, Nic is a pariah with a price on his head - rolling with Tessa and Imariel for the next few decades is going to look a lot better to most of them.

Deirdre's Fallen might be the only one willing to take Nic's calls at this point.

I really do want to see some silver buried under a wizard's creepy prison-fortress island at some point, though.

Doesn't the last Starborn predate Mab being in her position though?  When was the Last Starborn active, about a thousand years ago?  And Mab has been WQ since Hastings, was it?  Which is maybe a thousand years ago, but it's also off by 5%.  And that assumes that she jumps right into it immediately.

We don't actually know that Mab started the gig exactly in 1066.