Her biggest advantage is that she didn't have any unAsmodean thoughts until she was confident she wasn't being mindread; if you try to reproduce this, you'll get interesting results, but not a break like the one she really had. 

Her second-biggest advantage is the price of her soul in Dis. She can potentially command vast and unusual resources for her plan, which is good, since she'll need them. Devils, she suspects, have something like unimaginable Sense Motive for efforts to get one over on them while selling your soul; that's why this Carissa can't try it, even with her pin of Glibness, even with a better one. But a sincere, Asmodean Carissa....

Probably a lot of traitors with access to memory modification would try some memory gambit. She has a couple reasons to think it'll go better for her. The first is that she is not, in fact, asking to be sent to Keltham.

She is asking to be sent to Hell.

Still, if she imagines Aspexia Rugatonn, the imagined Aspexia Rugatonn is very suspicious.

Indeed.  It is obvious that among the potential reasons to erase your own thoughts is that you turned traitor, and then, used your +6 headband to try to steer your future self along a pathway that ends with you fleeing to Keltham in Osirion.  Even if you don't seem Atonable, maybe you've outgrown your previous nightmares, and are now ready for someone to statue you until Civilization brings Hell to terms, if need be.

Wherever your attempt to steer your future self ends up, such as with travel privileges, Rugatonn will ask if that was the point of the whole plan, no matter what cleverness you essay along the way.

She doesn't want to show her hand too much, how many escape plans she thought of; having carefully considered a dozen ways to escape Cheliax is also something traitors do more than people who aren't traitors. 

But it'd be easy. If she kills herself Osirion will learn of it and raise her before she goes to trial. If she calls in Olegario and tells him she needs an urgent Teleport to Egorian, he'll take her, and once outside the nonintervention zone she can send him home and pray to Iomedae, or Irori, or Abadar, or use a scroll of Sending. With the aid of some Suggestions she could probably get him to go along with defecting outright, or ask him to put on her Geas earrings. 

If she sleeps and then spends the rest of the night desperately trying to figure out how to trim down Teleport into something she can hang, giving up on preparing any other spells, she has a feeling she might land it by morning - either by finding the right shortcut or because desperately trying for hours to hang a spell is the kind of thing that might let you claw your way to fifth circle if you're close enough - and then she could Teleport under her own power.  And the Project is no longer staffed by some of the most senior wizards in all of Cheliax; she could probably just, Glibness pin up, ask for a Teleport scroll and get it, and then a junior Security could leave the project site with orders signed by Carissa Sevar, and read the scroll as soon as they were off-site.

She shouldn't say all those things to Aspexia Rugatonn, but one or two, maybe, at the end of the letter, so Aspexia Rugatonn can read it with growing suspicion and then notice her suspicion was unjustified. There's a mental technique, for un-learning properly, letting your mind relax to the level of suspicion it should be at and not less than that, but Carissa herself hasn't mastered it yet and she doubts Rugatonn has. 

Rugatonn is old.  Rugatonn is wise.  Do not underestimate her.  If this trick works at all, it will only be because Rugatonn decides to read through the letter at an ordinary high speed and not deliberately pause to think after each sentence before she gets to the next, only forming unconsidered reactions as she goes, and not considered ones...

...which it isn't implausible Rugatonn may do, she's always in a hurry.  Rugatonn's casting stat is Wisdom; seeing manifold ways to escape Cheliax is more the work of Intelligence.

All right then, Rugatonn is perhaps forming an unconsidered theory of Sevar's escape-to-Keltham motives as she swiftly reads, to be spiked at the end of the letter.  What's at the start?

Rugatonn obviously wants to know immediately why Sevar deleted her memories.  Was it, perhaps, because she had a traitorous thought, and hatched some elaborate plan to fulfill her traitorous desires without those desires being readable?

It would be nice if she could claim that, instead, she had a dangerous thought, and shouldn't specify it even in the document.
 

Too dangerous even for the Most High to read?

It sounds like a blatant excuse, and just what a traitor would write.  And yet, surprisingly plausible, given the mess with Peranza, tropes in play, and probably Rugatonn's own experience with a Thing or two out of the Dark Tapestry...

Sevar should write down this dangerous information, so that Rugatonn can very carefully and respectfully not read it herself and show it to Gorthoklek.

Yep, that's what she'd do too.

(Can she think of anything that would break Gorthoklek? It merits a little thought, at least, even though it's almost definitely impossible.)
 

If there's such things as thoughts that break powerful, coherent agents, they're probably not readily thought up by smaller, incoherent agents that don't have a model of the powerful agents in much more detail than them being powerful.

Fine. 

What could she have run into, that is dangerous and visibly so, that would cause her to do what she's planning to ask to do, and would not cause her to decide to overthrow Asmodeus?
 

So, just to be clear here, Carissa Sevar wants a thought such that:

- It implies she should go to Hell and buy up the souls of previous Project Lawful employees.  (If that's something you can even do, with Hell.)
- It doesn't imply she should overthrow Asmodeus.
- Carissa Sevar needed to erase this thought from her own mind and have it only be known to Aspexia Rugatonn.
- None of this is particularly what a traitor would try to do.

Well, this problem sure doesn't suffer from being underconstrained!

Also she has two minutes to both solve it and write it down. It's all right, it's easier than overthrowing Asmodeus will be.

She thinks she has the first part - implies she should buy up the souls of previous Project Lawful employees - figured out. She should do that because it'll create the slack in Hell's budget for her next request. 

She thinks she has the second part halfway - there are thoughts that'd be destabilizing to current!Carissa but not to a soulsold one, ones whose destabilizingness is conditional on her having a pathway out and no longer dangerous once she doesn't. 

- and therefore, which cease to be dangerous once she's sold her soul. 

- The tricky part is why she needed to erase it from her mind, but she has half an answer there too, it's something to do with the state of mind she has to be in for Irori to release his grasp on her. 

- well, at least she has going for her that probably no traitor, ever before, has demanded to be taken by the Most High personally to Dis so she can renounce Irori and sell her soul directly to Dispater.

This is legitimately not the cleverly disguised traitorous plan that Aspexia Rugatonn was expecting to see.

Hey, wait, what about Keltham?  Remember that Keltham guy?

Keltham is very sad about this soul-selling plan!  It obviously doesn't work unless you return the premium on his soul-repurchase option first!

He might get upset enough to finally really hit you and mean it!

...aaaand then he's going to destroy the multiverse specifically so that you can't go to Hell.

Well, no, he wouldn't do that, because he probably hates her and never wants to see her again, though if he does feel strongly about it, she will point out to him that her plan, where she overthrows Asmodeus, also results in her not going to Hell-as-it-presently-exists, and his plan leaves her vastly worse off than if he'd never existed and never met her and she'd never offered herself to him, and probably he should not make plans for her sake which have that property. 

His plan is very stupid but she doesn't feel contempt for him about it, just conviction that she'll be able to talk him around once she's earned his forgiveness and acquired sufficient resources for a better plan.
 

Oh right, that old model is out-of-date.  Keltham now hates her and never wants to see her again.  This is completely reasonable.

Really is. She kind of doesn't want to dwell on just how reasonable it is because that thought is both painful and unproductive.

Anyway. If she says she wants to pay Keltham back face-to-face whole thing looks like it's orchestrated to arrange that, but there's nothing in the contract that suggests she'd have to pay him back face to face, so she'll just send someone with the money.
 

Keltham hates her and isn't at all horrified by this.  He expected no better from her than throwing what was nearly a marriage contract back in his face.  If she sells her soul to Hell with no take-backs, good.

Still completely reasonable!

He's plotting to annihilate her. He broke the marriage contract first when he tried to destroy her utterly and everyone and everything she cared about. 

It's fine, Carissa!  Everybody just ends up in another universe!

You have limited time.  Stop thinking about Keltham.

Yep, on to figuring out something that could plausibly have threatened her loyalty to Asmodeus that Aspexia Rugatonn will be genuinely impressed by and that is less threatening if she's soul-sold.