It occurs to me that Jaime’s sulky act surrounding his regicide might have less to do with breaking his oath to avert a holocaust than with his own dysfunctional relationship with Tywin. Have we ever seen an account of Tywin’s reaction to Aerys’s death?

I don’t think it’s got a lot to do with his relationship with Tywin – Aerys ordering Jaime to kill his father is the catalyst that gets Jaime to finally act on his feelings, but the feelings that were already there before Tywin enters into it were primarily about the conflict between his romantic conception of knighthood and the horrific reality of serving a mad king who got his rocks off on burning people alive. 

As for Tywin’s reaction to Jaime killing Aerys specifically, I think he would have seen it quite similarly to how he saw the death of Rhaenys and Aegon:

“We had come late to Robert’s cause. It was necessary to demonstrate our loyalty. When I laid those bodies before the throne, no man could doubt that we had forsaken House Targaryen forever.” (ASOS, Tyrion VI)

For Robert to become king, Aerys needed to die. Jaime killing Aerys was another example of the Lannisters demonstrating their loyalty to the new regime, and was thus the necessary and practical thing. 

OTOH, I think Tywin would have felt very differently about what Jaime did after. Sitting the Iron Throne ahead of Robert could have jeopardized the Lannister-Baratheon alliance; if you’re going to sit the throne, you only do it if you mean to rule and have a plan that you’ll ruthlessly pursue to make it happen.

And so on. 

About Pycelle and Varys. How is it that Jaime and the Pyromancers knew about the Wildfire under KL but Pycelle and Varys didn’t? Pycelle obviously didn’t know since he wanted to let Tywin through the gates and he wouldn’t consciously endanger him. Varys doesn’t seem to know either. Why did Aerys hide the plot from them? If he didn’t trust them about the Wildfire, why did he turn to them for counsel when Tywin knocked on the door, and then deferred to Pycelle anyway?

“Three can keep a secret if two are dead.”

Here’s who Aerys told about the wildfire: the three pyromancers Rossart, Garigus, and Belis. That’s it. (Jaime Lannister was the only other person in the room when this happened, and he killed the other four people involved.) The Lord Hand Chelsted “became suspicious” when he saw “Rossart, Belis, and Garigus coming and going night and day,” but when he confronted Aerys, he was immediately burned alive and couldn’t share what he knew. 

So Pycelle didn’t know because Aerys kept his Small Council ignorant of his plans, because he didn’t want anyone trying to stop him. 

As for why Varys didn’t know, even the best spymaster needs time to develop sources, collect information, and then correctly analyze this information. The problem is that the wildfire plot only included a few people, who maintained a high degree of operational security:

“Everything was done in the utmost secrecy by a handful of master pyromancers. They did not even trust their own acolytes to help.” 

(Jaime V, ASOS)

As for why Aerys didn’t trust them on the plot but trusted them on other things, this was a fairly consistent pattern of his paranoia, where he was simultaneously afraid of the same people he was deeply dependent on, and refused to remove people who he thought was trying to kill or replace him:

“By this time, King Aerys had become aware of the widespread belief that he himself was but a hollow figurehead and Tywin Lannister the true master of the Seven Kingdoms. These sentiments greatly angered the king, and His Grace became determined to disprove them and to humble his “overmighty servant” and “put him back into his place.”
…Tywin Lannister attempted to return his chain of office the next morning, but the king refused to accept his resignation.
Aerys II could, of course, have dismissed Tywin Lannister at any time and named his own man as Hand of the King, but instead, for whatever reason, the king chose to keep his boyhood friend close by him, laboring on his behalf, even as he began to undermine him in ways both great and small.”

(WOIAF)

Aerys’ weird mind-game with Tywin lasted for a decade, and then the moment it ended, he started a new one with Jaime Lannister. As Jaime puts it, “Aerys liked to keep me close. I was my father’s son, so he did not trust me.” So it’s entirely consistent that Aerys would feel the same way about Pycelle and Varys.

One more time, re: Aerys and public works

Having twice had a rather long answer to an ask about whether Aerys’ grand schemes had any merit gotten eaten by Tumblr, I’m stubbornly trying again. 

So here’s the thing about Aerys’ grand schemes: they all fall victim to the same flaw of monumentalism. They’re all huge, incredibly expensive, and very difficult projects ultimately chosen more to satisfy Aerys’ grandiose self-image than they are on their individual merits, and they would eat up resources that would probably be better spent on more prosaic, but also more useful and more likely to succeed projects:

“His Grace was full of grand schemes as well. Not long after his coronation, he announced his intent to conquer the Stepstones and make them a part of his realm for all time. In 264 AC, a visit to King’s Landing by Lord Rickard Stark of Winterfell awakened his interest in the North, and he hatched a plan to build a new Wall a hundred leagues north of the existing one and claim all the lands between. In 265 AC, offended by “the stink of King’s Landing,” he spoke of building a “white city” entirely of marble on the south bank of the Blackwater Rush. In 267 AC, after a dispute with the Iron Bank of Braavos regarding certain monies borrowed by his father, he announced that he would build the largest war fleet in the history of the world “to bring the Titan to his knees.” In 270 AC, during a visit to Sunspear, he told the Princess of Dorne that he would “make the Dornish deserts bloom” by digging a great underground canal beneath the mountains to bring water down from the rainwood.” (WOIAF)

Starting with the worst ideas, trying to conquer the Stepstones or to fight a naval war with the Braavosi for the hell of it are both terrible ideas. As we saw with Daemon’s War for the Stepstones, such conflicts would be very long, very expensive in blood and treasure, would be unlikely to succeed in the long-term due to the difficulty of maintaining forces abroad, and would definitely see the Iron Throne drawn into a broader Essosi war as the various Free Cities reacted negatively to the aggression of their neighbor. 

Next tier down, building a new Wall in the North would be hugely expensive to construct, would definitely involve a brutal war against the wildlings, and at the end of the day would gain you 300 miles of territory that’s mostly non-arable let alone fertile. By constrast, resettling the Gift would effectively extend the kingdom north by some 150 miles, would allow for the expansion of production due to the superior arability of the land south of the Wall, and (by expanding tax revenue coming out of the Gift) would allow for an expansion of the Night’s Watch which at the moment can barely staff the Wall they have let alone a second one. 

Similarly, building a whole new capitol down the road, a la Springfield in the Simpsons episode “Trash of the Titans,” is horrendously expensive, because you’re basically taking a complete loss on all of the built-up real estate in the old capitol. By contrast, investing in public infrastructure in King’s Landing is somewhat prosaic, but better water and sewage systems, paved streets, a public sanitation department, etc. would be a much more efficient solution for the capitol’s miasmic probems. 

Finally, while expanded irrigation of Dorne is key to its economic development, trying to dig a tunnel through the Red Mountains and then lay an aquaduct through that tunnel is almost certainly beyond the means of the Iron Throne. It would be hard enough to divert the Torrentine, allowing for the irrigation of the western third of Dorne, but that project would be far easier than trying to tunnel through hundreds of miles of rock by hand.