I ran Carcosa pretty much as it appears in the Tatters of the King campaign. I described the off-kilter changes to the world and kept James moving through it. In the end, the group got divided because Charlie slipped off the narrow bridge into the water. James followed her but couldn't find her until he came out of the water and saw her on the steps. There was no sign of the other two (Peter Walsh and Joy's retainer, Juliette) as Carcosa had already spirited them away with its strange dream logic. It was difficult to get them back together as James' suspicion towards the various Carcosan people meant that he rebuffed any of the NPC's attempts to help him. So in the end I made him stew for a bit before he stumbled across the museum where he met with Alexander Roby and, later, the other two NPCs.
As he was all on his own, at the very end I gave him more time to plot out how he would react to the ceremony than I otherwise would. He didn't have several minds to remember the bell, after all, so I thought it only fair to give him a chance.
When Hastur appeared, Yolanda ran away as James had been rather callous toward her (thinking her a figment of a dream world or some supernatural creature) so she took her chances on her own. Unfortunately, she ended up having her throat slit outside the garden labyrinth but what can you do? Easy come, easy go.
While the actual gameplay didn't look any different, I did have a different cosmology in mind which could have led to a few different changes that didn't end up coming up because of the player's choicse being pretty standard to the campaign.
The people in Carcosa are Changelings who have been delivered by other True Fae as according to ancient arrangements with the King in Yellow. Over time, their masks become affixed to their faces, though none of them believe this is true until the Stranger arrives. In which case, when he is due to remove his mask and can't, they each attempt to remove the mask and realise the same laws apply to them.
Yolanda is an exception to this as she hasn't been here for very long. Neither has Noss. These Changelings don't know of True Fae or their own status as kidnap victims. No one else could give them that knowledge and they're minds have been freshly scrubbed. Most of those taken to Carcosa were taken as small children and have no understanding of any other world so escapes are rare. Time passes strangely on Carcosa too (when it's not cojoined with Earth) so most have been there so long that there is nothing left for them to come back to. No anchors to their memories. Although this is an Arcadian Realm, Carcosa itself will make the requisite agreements with the Wyrd on visitors' behalf to allow them to walk in the location.
Humans who wear the masks given by Noss will slowly find it attaching to them over the process of several months as the Wyrd adapts the mask to become their face.
All humans who enter this realm and wear masks or eat and drink while here become Fae-touched mortals, a lesser supernatural template that can be replaced by ghouling, or more permanent transformations such as vampirism of the First Change. Remaining here for much of a week will also become Fae-touched. This could have happened to Charlotte and Juliette, but it didn't, because James was suspicious of the masks and they were suspicious of the food and drink. James did end up drinking Changeling blood in a frenzy but he can hardly take on a lesser supernatural template. Normally the player can be quite quick to identify what he bit but my solo player makes no such assumptions when he's in a Call of Cthulhu game. It's interesting, because he knows it's all been converted to World of Darkness logic, yet still he makes no assumptions. I find it adds to the horror.
Anyway, if your World of Darkness character is human and arrives in Carcosa, you could grant any who eat, drink, or wear a mask the Ensorcelled Mortal merit at two dots as Fae-touched individuals are permanently ensorcelled. However, I recommend re-naming the merit to 'Touched by Carcosa' so the players don't know quite what is happening. They might then assume that Changelings they see are just strange 'hallucinations'. So long as they don't meet the same 'hallucination' twice and work out that it's oddly consistent, they probably won't figure it out as a Changeling.
In this campaign, it's unlikely that any of the player characters will become Changelings here. Perhaps if one were abandoned in Carcosa, then perhaps, but such an event would render that character unplayable so the players would never know. In the latter half of the campaign, the player character who led the King in Yellow astray to Carcosa and became the new 'Stranger' will become a Changeling. As they can escape after a year, the player might then pick up a rather low Clarity Changeling.
Since Changelings don't necessarily know that there are more of them nor about the details of other True Fae, both the player controlling the Changeling and the others might not even figure out their friend's true nature for quite some time. One thing I would suggest, though, is to give that character a working knowledge of pledges alongside their own Contracts as that's pretty innate to anything partially of the Wyrd.
If you'd like to read more articles about the Tatters of the King conversion to the World of Darkness and the campaign that has sprung up around it, have a look over here.