Okay, let's go back to the original challenge for a moment. I submit that my dragon and his cult have completed this challenge.
* The dragon does not have innate magic or spell like abilities. Just his claws and breath weapon. Eventual minions are allowed to use magic.
Check.
* The dragon is allowed to use magic items (not custom made), but the amount of magic items should be kept to a minimum (PC loot).
The only magic item my dragon has is a ring of invisibility, which is well within the means of a CR 23 monster with triple normal treasure.
* Assume that dragons in this world have no inherent alignment.
Check.
* The dragon must be an antagonist for good aligned PCs.
Most certainly.
* The target is a "normal" human kingdom with nobles etc.
Check.
* Either use a Adult, Old or Ancient red dragon (Say in your post which one. The older the dragon the more extensive its intrigue must be).
Ancient red.
* The dragon should at least be able to influence the ruler of a big city. Leaders of powerful guilds, nobles or even kings is preferred (and for the ancient dragon required).
Some of these people are in the cult. For those who aren't, the dragon's bard minion can influence these people directly via Diplomacy and Bluff. The dragon can also manipulate events to force the actions of human leaders--e.g., using cultists in the Imperial bureaucracy to delay food shipments into the capital, thus inciting food riots and compelling the Emperor to recall some of his soldiers to restore order.
* The Dragon is the BBEG, not the minions. So the minions should not be collectively be much more powerful than the dragon. (If the minions can put together a 4 man adventurer party which can kill the dragon I consider this as too powerful).
A kobold tribe; a 12th-level sorceror who's devoted most of his slots to things like greater anticipate teleportation, teleport, and lesser planar binding; five erinyes devils; and an 18th-level bard; against an ancient red? The dragon would mop the floor with them.
* The dragon has enemies. Expect adventurers to attack from time to time. Explain how the dragon defends itself (also include tactics like Scry&Fry or bypassing the dragons defenses and CdGing the sleeping dragon)
Scry-and-fry and coup de graces are dealt with. Unless the adventurers are tough enough to overcome an ancient red in combat, the dragon is fine.
* !Important to me! The dragon does not live in a void. Explain where eventual minions live and where they get their food and resources from. Explain how the dragon keeps contact with his agents and people he blackmails. When the dragon uses charged magic items explain where he gets new ones.
The kobolds hunt small animals around the dragon's lair. The erinyes act as messengers between the dragon and the high-ranking cult members, especially the bard, who lives on money from the cult. The cult provides a steady inflow of gold, which allows the dragon to make occasional expenditures out of its hoard. The kobold sorceror teleports the bard around when needed, and retrieves the cult's deposits to the First Bank of Dragon. The dragon does not generally use charged magic items.
* The world is not static. Other political NPCs and maybe even the PCs have also political ambitions. Explain how the dragon can stay ahead of them (especially when those (N)PCs use magic for communication etc.)
The dragon stays ahead the same way any backstage manipulator does, by outthinking his foes and outmaneuvering them. While he does have the drawback of not being personally "on the scene," he compensates by having the combat prowess of a dragon at his beck and call. This enables him to do on a small scale what the Shadows did with the Centauri on Babylon 5...
Bard: "You know that rival family of yours?"
Noble: "Yeah."
Bard: "Suppose they didn't have those iron mines in Westmark. That would make things much easier for your trade factors, wouldn't it?"
Noble: "I suppose it would. But how--"
Bard (with a mysterious smile): "I'll see that it's taken care of. Let me know if you find any other trifling matters vexing you. Who knows, I might be able to assist you with those, too."
Sometime that night, the iron mines are obliterated, leaving no survivors... and no corpses. (The dragon, mindful of spells like speak with dead, had the erinyes haul the bodies out so he could eat them.)