The author's clues are pushing in two different directions. "Taboo tradeoffs" in the title, and that Harry's Dark side is delivering the solution, implies an answer that is morally unnerving or at least cold-blooded.

The author's line about Harry shifting from seeing the Wizengamot voters as "wallpaper" to seeing individuals with agency, "PCs", and the line about remembering the laws of magical Britain, implies an answer that involves the incentives and 'rules of the game' of the other Wizengamot members besides Lucius and Dumbledore.

None of the solutions I've seen (let alone the few I've thought of) seem both Dark/taboo and social/voters-are-PCs.

Harry calling in the (nominally) Imperiused voters' debts: clever, invokes customs of magical Britain, makes the voters PCs rather than wallpaper, but not very Dark or taboo.

Harry threatens the crowd with the Dementor somehow, or browbeats Dumbledore into ruining his reputation: Dark/taboo, but doesn't invoke the customs of magical Britain or treat the other Wizengamot voters as PCs.

Is there a solution that both invokes magical Britain's laws / makes PCs of the voters and involves some alarmingly Dark/taboo move or trade by Harry?

Boy-Who-Lived marries Draco Malfoy?