Does not answer to the Sids. At all. In fact, it's leadership dislikes them.
To which they also have no access. Hu Dai might have some, but good luck convincing her to let them into Creation, or activate any there.
Which is obvious as fuck and takes time in which they can be stopped. Also, there are very few 250 year old DB's, fewer with War 7 E7. Like, probably a count below 20, and some are probably decrepit, others never bother to learn the charm. There might be 10 people capable of using the charm in realistic circumstances in all of Creation. A Thousand Forged Dragon is a more realistic threat.
Not really. Torrential Cascade is the only potential large-scale damage dealer. Wheel of the Turning Heavens could transfix a city, but if anyone is hurt it ends, so less good for destroying a city. Summon Warstrider is a big weapon, but again not an existential threat to a city in any rapid timeframe. Sapphire Circle has a lot of nice utility, and not much cataclysmic destruction. 2CDs are your only common option, but again, most are not city killers in any rapid frame of time.
If you think this does not result in an immediate massive response from everyone you are an idiot. If you think there are no other safeguards you are an idiot. This is not going to happen.
So I take it you agree that all those methods of mass destruction have corresponding warning signs, countermeasures, and/or personal risks? And further that the Sidereals don't have anywhere close to unlimited access to the resources of Heaven, because many celestial gods would gladly seize any reasonable excuse to harass or prosecute them?
Sidereal astrology works by modifying the Loom of Fate, which is how the gods plan Creation's future. Thaumaturgical astrology works by reading those designs in the night sky, which is a 'display screen' for the Loom. Therefore, mortal astrologers can - and for extreme modifiers, probably will - notice incoming astrological curses or blessings, even if they don't . Warning signs, check.
More essence users becoming directly involved could end the destiny early, just by the fact of their involvement, the "essence burden." If enough mortals pray to the gods for relief from the astrologically-induced problem, that translates directly into political pressure on the sidereals who laid the curse. Countermeasures, check.
A wide-ranging astrological effect can't activate until all the Essence 2+ folks in the area of effect have been touched by the sidereal. Gods and elementals are exempt, but
only if they're obeying the laws of Heaven - rather than, say, slacking off, or extorting extra worship, as all too many of them do in this fallen Second Age. That means, if anything at all
interesting is going on in the area, the destiny CANNOT be "launched from orbit" - gotta visit the site, go around shaking hands or something. As an alternative to a personal visit, there's Predestined Delivery Shaping, but that adds further delay and risk of detection. Personal risk, check.
As for examples of city-wrecking sapphire circle spells, Cantata of Empty Voices and God-Forged Champion of War were already mentioned. A warstrider can do significant harm to an urban environment
by accident, surely intentional and systematic destruction would be worse. The extent to which a single second-circle demon could singlehandedly wreck an otherwise credible city in any lasting way is debatable, but someone like Gervesin or Octavian could at the very least kill a lot of people and make a long-lasting mess just by going door to door, while many others like Gumela or Vorvin-Derlin (not technically a second-circle demon but summonable by the same spell) have unpleasant effects on infrastructure and/or public morals.
There's also The Crumbling Walls (which has literally no other function), Geyser of Corrosion (which would likely require either multiple castings or strategic placement, but interrupting a major road with a caustic pit is no laughing matter from a logistical standpoint), Hideous Confusion of Tongues (it was good enough for the Tower of Babel), Magma Kraken, The Princes of the Fallen Tower (again, barely any purpose beyond large-scale demolition), Summoning the Heart of Darkness ('entire armies have slaughtered themselves'), Swift Spirit of Winged Transportation (grab an important building, drop it from a great height), Torrential Cascade (small structures destroyed outright, area may take weeks to return to normal if it ever does), and Whirlwind of Fate (insofar as it could facilitate a commando raid, arson spree, or other mundane destruction).