I would expect the retired Queen to be Mother Winter. I believe she gave or lost her walking stick to the Councils "Black Mage".

The decline I think was a general movement of globalization and less and less tribal-nations, etc.  Or, the early beginnings of the the ascent of Reason and Scientific thought/observation.  Or something else entirely, like a Starborn causing a ruckus.

One would wonder why an uber-powerful entity like Hecate WOULD split her power up?

Either way I doubt it's as integral to their organization, as it is with the Summer/Winter courts, but then the Summer/Winter system was very specifically designed and built by their predecessors.  And I still have no idea how the Stone Table related to the Wyldfae.

The coins are all about choice.  That is why a fallen cannot just put a coin in your hand to corrupt you.  It does not work that way.   I suspect that choosing to give up the coin operates the same way in reverse.  It blocks the person from calling the coin.  I suspect the only people who "could" call the coin are people who choose to take a coin, but have not yet given it up.  Even other fallen could not summon a coin.

One of the early reasons for the adoption of firearms as weapons, even aside from their power, was that you could learn to use one effectively more quickly than you could older weapons.  That was part of why guns were in use even when they still weren't all that much more potent than blades and bows yet.

But some things, like activating a circle around the roulette wheel before you drop the ball, are going to be both subtle and effective.  Beyond that the same mentality for security applies to both magic and science: they just have to Detect.

I suspect the magic secrecy dam is really under strain right now -- and will break at some point (perhaps at the start/during the final trilogy).