Transparency in Skill Challenges

[Skywalker]

And some players might try to just have one Face character make those 4 diplomacy checks in a row to brute force the talk without ever dealing with the powerplay, while the rest of the party idly watched. Which would be pretty awful, but by RAW follow the complexity and rules of the challenge in some cases.

Understand. I don't think I have ever encountered this as the tools are there to design your SC to avoid it.

Looking at my basic example:

First, only 1 milestone relates to using Diplomacy. As such, it would likely by max 1 success. The Face PC could talk and talk all day but he couldn't win the NPC over more than once*. The overcoming cultural differences milestone could be done with Diplomacy if a player came up with a good narrative explanation. But I would probably make that a Harder DC or simply a +2 bonus to another, and encourage use of a more appropriate skill like Insight, History or Streetwise with a lower DC.

Second, 2 of the 4 milestones are not really achieveable by the Face PC engaging the NPC with Diplomacy. They relate to investigating and revealing a power play that would be taking place between other people in other places. As such, the other PCs have to step up or the party would not succeed.

*If the entire Skill Challenge is just about winning a single NPC and can be done by Diplomacy alone, then make it a Skill Check, not a Challenge.