...but the Malik has to state and swear to the use of those Wish diamonds, as must be agreeable to his customers; there are many non-destructive uses of Wishes that they'll be happy to agree to.

How many Wish diamonds does she have to trade?

"Two I'd considered for purchases here; some others, for some other purchases, which I'd consider if you have more than I'm aware of."

He has but the two Ioun Stones, but there is much else about his shop that might interest one who pays in Wish diamonds.  Since he has bound himself not to say anything of it, he admits himself quite curious what might lead Dispater to open out His treasury so.  Perhaps, if he knew her purpose, he could point her towards more appropriate items.

She's starting a cult. She mostly wants to look really cool and conceivably at some point overthrow some governments.

...

The Malik shows her a variety of items, none of which are truly tempting except for a Robe of the Archmagi.

It's gray.

Neutral-aligned.

Carissa would have to Atone to Lawful Neutral to use it.  But she keeps up a Mind Blank, so Hell wouldn't find out, right?  "I wanted to be able to wear a gray Robe of the Archmagi" is something that any reasonable deity should understand as a just reason to Atone.

....she is briefly tempted, but, no, she thinks that there is absolutely no way a visibly Lawful Neutral person could serve the specific role that Carissa Sevar is serving. She could disguise it, obviously, but some people cast True Seeing, in situations involving her. 

Also she doesn't actually want to go to Axis. If all this ends, somehow, with nothing fixed or improved or destroyed, with all who trusted her betrayed, then she should burn eternally; that would just be correct. 

Not that she even could go to Axis, but it seems like if you don't even want to you probably can't Atone.

She'll just get the two ioun stones.

He'll try for a while to bargain her up to three Wish diamonds before grudgingly settling on two.

These gems, he'll swear to use only for increasing his own abilitystats.  But he desires not to do so this day, for he has hopes of coming across a further Wish diamond sometime in the next millennium, by which means he might increase a stat by three rather than two.

She'll purchase the Robe of the Archmagi for a friend, if he agrees to use those three Wish diamonds tomorrow, but not today, and to swear to total secrecy the Maliks whom he pays, or offers to pay, to help him cast three Wishes in succession.

...that Robe of Archmagi is really worth two Wish diamonds.

She has less patience than does Dispater's.

Do they by any chance have a fourth Wish diamond they'd be willing to exchange for... basically anything reasonable he has?

If it'll save them bargaining time with other Efreet who'll otherwise waste that time, she's willing to look over any easily-portable valuables he has, such as would be readily accepted as currency within the City of Brass, that they could later spend in place of the last Wish diamond, and not need to repeat all this bargaining.

They get out of there eventually, Tarnish having sold a fourth Wish diamond for noticeably less than it was worth as of today, and rather a lot more than it might be worth later.

There are some people in the world who'd know Carissa Sevar well enough to infer, from how much she doesn't look happy about her ioun stones, that either she has gained a lot of skill at bluffing or she thinks something incredibly awful is going to happen very soon.

Next up, the Wishes, then?

Yes, they do in fact have Things To Do and people to become.

Walking through the City of Brass with a lesser artifact crown, plus two fucking IOUN STONES orbiting your head like you're fucking Felandrial Morgethai, attracts more stares and avaricious glances than just the artifact crown.

Nobody tries it; at least, not yet.  Everybody in this part of Fommok Madinah is a few millennia old at least, and didn't get to be that way by directly attacking unreadably Mind Blanked adventurers with a lesser artifact and two fucking Ioun stones.

In time, they succeed in coming across a larger building, a mighty brazen slave-market, which also advertises in passing that it grants Wishes, or even five Wishes in succession; as is one indirect way of advertising that this business is worth the attention of at least five noble Efreet.

In retrospect, you wouldn't actually expect to find a shop that was just five noble Efreet waiting around to perform a service whose custom they might receive once per year if they were lucky.

Is there anything good in the slave market? Carissa is in the market for ways to piss off her ex.

Five Wishes, please.

It's a little more complicated than that but, yes, she can get to that point under Keltham's onerous conditions without spending too much time.  Two fucking Ioun Stones orbiting your head commands a lot of commercial respect.  A devil-wrought lesser artifact is something you get by being on sufficiently good terms with Hell; Ioun Stones take money.

The interior of the building is over-warm by Golarion standards and very temperate by Elemental Fire standards, replete with air she could breathe without even a Planar Adaptation.  There are mortal slaves here, drawn from the Material, who require more favorable living conditions.  Though Carissa was only permitted to see a single chamber of slaves drawn from Golarion and the other planets of its star-system, for some reason or other; still, she could have had a Shobhad slave-warrior of Akiton, or a pliant and broken Lashunta of Castrovel, did she wish to piss off Keltham by buying such a person and then, presumably, mistreating them.

Mentioning that she wants to engage in secret negotiations with the noble Efreet owners will garner some looks even more arrogant than usual; and the Malik brought forth won't, in this case, agree to secrecy only on the basis of intended purchase.  But statement of intended purchase, and 'token' payment of something worth around 2000gp, will do.

A sixth Wish diamond is sufficient to pay for the casting of five Wishes, even under the condition that the Malik must state and swear to its use:  One of their number will hoard it through ages until he has assembled enough diamonds to raise the final abilitystat he requires to ascend into the next rank of Efreet nobility.

She will eventually end up in a windowless inner chamber of this place, as fiery as the rest of the interior was temperate, orange flames replaced by white flames shot with blue; hot enough that Carissa will feel it through the Planar Adaptation.

Carissa Sevar is obviously a very wealthy person, by the way.  And now she's in a windowless chamber with five noble Efreet who might, possibly, be tempted by walking away with six Wish diamonds and two Ioun Stones in profit, rather than just the one Wish diamond.  Five Malik acting together in a surprise attack - not that much surprise, but unannounced, at least - and supported by their subordinates and slaves shortly after, might take a victim unprepared even if they were relatively mighty for a mortal.

That person at INT 27 and outlining a dozen possible worlds will have considered Carissa's death to Hell and its mind-reading as something Carissa might want consciously and subconsciously.  Carissa Sevar won't have been allowed to come on this trip and augment herself without a promise that she'll try by sincere effort and wit to stay alive, however much she might hope to die.

That person will not have asked of Carissa that she not hope.  But he'll have required that Carissa not act to bring about her hopes, that she if necessary act to prevent her hopes from coming to pass.  He's put himself together in a way where he can hurt about that, about demanding things like that from somebody he still cares about; he no longer tries to avoid that hurt by taking refuge in calculation.  But the reasoning will be clear nonetheless, and he will have demanded of Carissa what he must, to ensure that this universe can still be destroyed if necessary, for there's people hurting more than her.

So what preparations are about Carissa, then, against the greed of these Efreet?  Especially any visible preparations, as might discourage their greed from being acted-upon in the first place?

Those preparations cannot involve too many protections or displays that are both expensive and temporary; their pair needs to repeat this task eight more times at least, before the trip is done.

Intelligence is coming up with a ridiculous stack of Moment of Prescience and Contingent Gaseous Form and Anticipate Peril and Foresight and testing in advance that she can Banish herself using her crown when on other planes and making Greater Slaying arrows for the efreeti-bane crossbow Tarnish is conspicuously carrying.

Wisdom is realizing that all of that has at best an eighty percent chance of working given the known capabilities of the efreeti, and that the efreeti too might have unknown capabilities, and just negotiating up front for their safety during dealmaking. It makes her look slightly less invincible, but it also makes her actually safe. 

The Malik will of course act terribly insulted and disdainful, but eventually give in about this.

(Carissa is the only one on the expedition who has a hard version of this problem; Keltham can just have an "alchemical dead-man-switch suicide bomb that would destroy the diamonds" about his person.)

(Carissa made an attempt to persuade him that the efreeti would not especially believe him about this and so it was likely to be tested, but eventually gave up. If Keltham wants to melt himself and his possessions in an acidic slurry who is she to object, really.)