It's not too bad, really! Mostly the effects Hell has on you if you're behind a Mind Blank and Protection from Evil are not supernatural. 

He doesn't cast Planar Adaptation.

Probably this person is fine; he already knew this was true, had already seen the effects from closer range than this during Vision of Hell.  This doesn't represent new information, so there's no reason this person would be updating on it if he were thinking coherently.

(It doesn't actually work like that, and obviously he knows that, but sometimes it helps to remind yourself of it anyways.)

There aren't, actually, many tormented petitioners lying on the ground in this area of Avernus; even a very tormented petitioner would probably move away from a grandiose fortress full of pit fiends, if they could.

It's possible Hell is warned of her betrayal and is willing to spend the intervention budget to inform the Most High. Time is precious. Carissa knocks on the gate with a Magic Missile, for lack of something cleverer.

And these gates fly open, with none of the deliberate slowness of the fortresses whose duty is to defend the boundary between Avernus and Dis; this fortress is meant to be an imposing and suitable home for its hosts, not a bulwark against the invasion of the second circle of Hell.

"What business brings you here?" asks a hissing, hideous thing. 

"I have fifteen Wishes mine by right, out of Hell, and intend now to use them, in Asmodeus's service and Dispater's and my own."

There's a moment of silence. 

Likelier in the world where they know, already - where Dispater has in fact already withdrawn his favored possession status - although, would He, or would He amusedly wait to see how it played out - devils don't want to die but they also don't want to live, Keltham said, she needs to think about that claim more at some point -

The pit fiends teleport in. Fifteen of them, of course, because each can only cast Wish once per year. Mind Blank is holding back the fear auras, but that just means that anyone present will feel only the ordinary instinctive human fear at a being much larger than you and much smarter than you glaring down at you with contempt and distaste, surrounded by fourteen of its allies.

Carissa is surprisingly unterrified. Feeling incredibly confused about who you want to win is great for that, it turns out. If there was some error in her bargaining for safe passage out, she couldn't win this fight in a million years, so she didn't grab Keltham's scrolls with the intent of winning it at all; just of being memorable, and clearly powerful, and clearly in control. Her Unseen Servants are busy about her feet, shining the floor where she stands; her Dancing Lights have scattered to the ceiling to provide the most cinematic possible lighting. 

"STATE YOUR WISH, MORTAL," one of the pit fiends booms. 

- she's going to read the standard wording off a piece of paper with large blocky letters, impossible to misread. She has a very close to perfect memory, but being cinematic is not worth a one in ten thousand chance of saying something slightly wrong.

"+3 Intelligence, +4 Wisdom, +5 Splendour, reserve three," he says rather suddenly and before she can begin actually reading.

- that's fine and reasonable, and has the substantial advantage they'll have Hell-interpreted Wishes later if they need them, but she wishes he'd told her earlier so as not to mess with her cinematography. 

(Does it matter? There's a set of possibilities under which it matters, but she hasn't actually had time yet to think how likely they are. She may in fact just be doing this because it is better than feeling like all this is just happening to her without her steering.)

She'll tell the devils to give him +3 Intelligence, +4 Wisdom, +5 Splendour.

He doesn't outwardly react.

- okay not to be an embarrassing stereotype of herself but HOLY SHIT WISHES ARE THE MOST BEAUTIFUL THING SHE HAS EVER SEEN

In the time it takes them to cast the twelve Wishes Carissa has come up with a spectacularly clever rationalization for why the way to win from here involves her becoming a ninth circle wizard and casting all required Wishes for the project herself.

It's not true but it's so convincing. She'd be so good at it. 

How could anyone possibly think a universe was worth destroying that had such beauty in it?

When the magic has settled around Keltham and the air no longer glows she channels Abrogail, and gives him a little bit of a condescending smile. 

And then departs the fortress with him, her path cleared of dust by her Unseen Servants, and opens the Gate to go back. 

The man goes through first, expression still unchanging while the Gate is open.

Her expression, too, changes considerably once the Gate is down. "Why three???"

"I don't know if it was the right thing to do.  I realized - mostly from meeting you, only a little from Hell - that I am not holding myself together well enough.  More balanced stats for a time might help.  Planning for next round of augmentations to be slower.  More Constitution than 19 may not be dispensable.  Strength and Dexterity may mean things.  Would need to take risk and visit City of Brass anyways.  You might have a plan that only works with Hell-intepreted Wishes.  If the Wish-resource method fails, we have three Wishes for the last step of the current plan even if the pit fiends might pitch a fit about keeping the deal.  It costs extra time at INT 27 instead of INT 29 and maybe that difference will ruin everything but -"

"Sometimes you throw out the calculation and go with your intuition.  It's just important to understand the calculation first."

"Yeah, all right. I - failed to explain all my reasoning, here, which is on me." And they're not in the secret area, so she can't explain it now either. 

"Uh, that aside, I think enhancing yourself more evenly and more slowly sounds like a good plan, and I'm glad you agree that you're not holding yourself together well. ...no offense."

"I apologize for the sudden surprise.  It came to me at the last minute.  I'd have spoken earlier if I'd thrown away my calculation earlier."

She starts heading towards the more secret base sectors so that she can ask the rest of her questions. "Shall we, now that nothing's time sensitive on the scale of hours, take some time off and then reconvene? Do you have a summary I can read of your plans and reasoning relevant to them?"

"Taking time off sounds like a good idea.  And, no, I wasn't - expecting you to come back to me, before the primary plot had reached a climax.  Other people here for harm reduction - or even assistance - were not ones I considered worth arguing with about the core plans."

They pass through the 'inner' Forbiddance, then the 'core' Forbiddance, continuing to head to the demiplane.