I decided to make it a SC rather than a single check, with the SC requiring 7 successes before 3 failures and ultimate failure leading to combat...
Conversation ensued and every time the PCs made a persuasive point I would allow them to make a Persuasion check.
Given that this was one of the lead BBEG in the module, I made the DC 15 (Moderate) or 20 (Hard) depending on how well they had argued the point.
I would only allow a PC to make 2 consecutive arguments before requiring a switch, that way ensuring it was not just one PC with the best CHA stats dominating the encounter.
This is literally the problematic early adventure 4e skill challenge in a nutshell.
Skill challenges should be arranged in such a way that everyone in the party organically participates because they have incentives to do so and no one PC can dominate because there's nothing to dominate.
Sure, the Cha PC might overwhelm Persuasion, but there might only be a benefit from rolling Persuasion twice. Pretend for a second that we're talking about the party talking to a Duke. The Duke will refuse to talk to them unless they entertain him. Anyone who specifically entertains him will be considered too low class to Persuade him. Also, the Vizier is trying to manipulate the Duke. The Duke's wife has taken 'ill' from poisons the Vizier is giving her.
Entertain a Bored Stressed Out Duke: make an Athletics, Acrobatics, Sleight of Hand, or Performance check. That PC has the DC of all Persuasion checks by them raised by 5. Doing this opens up one Persuasion check.
Duke's Ill Wife: If no entertainment first, party must distract the King and Vizier to allow a healer to look at them. Stealth, Sleight of Hand or Deception check. Upon a successful Medicine check, this then again opens up one Persuasion check.
Something Funny Going On Here: Make an Investigation, Insight, Perception to realize how the Vizier is manipulating the Duke. This then opens up an option for a PC to distract the Vizier while the Cha-PC is making one of the above Persuasion checks.
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See the difference? The party has a variety of organic incentives to figure out things. You can even turn parts of this into a full-blown encounter or sets of encounters — maybe the Something Funny Going On Here is a whole game day in and of itself, where the PCs have to track down the maker of the poison, say a high up member of the Thieves' Guild and get them to be willing to confess to the Duke. Who the Vizier is then so distracted by that the Cha-PC gets their chance to talk. Or the Entertain a Duke requires convincing the local Circus to perform in front of the Duke and let one of the PCs be a member of them. Perhaps the local Circus has a problem needing solving? Or the Duke's Doctor needs some rare herb to be obtained before they'll allow the Medicine PC to look at her.
Run them all and the party might need an entire 3 game days to get the climactic scene of confronting the Vizier with his treachery. And even then, he has some twists ready to go, putting the skill challenge in play...