Time to sit down and go through that conversation – mentally, because he does not want any notes on it floating around, though he'll write down Caris' suggestion for conditions – and question everything he was thinking!
At the start, Caris offers him the opportunity to speak through an intermediary. He declines. He - doesn't think declining was the wrong decision, and in particular he got some key information out of it.
(- if it's true - he meant to ask for a way to verify it that doesn't run through 'sending someone to question Altarrin, who might instead get mind-controlled into saying whatever Caris wants them to', though...he's honestly not putting very much weight on the strong version of the contagious mind control theory, and if it is true then he's not sure that even failing to prevent the Office of Inquiry from staging a coup is enough to save them -)
Fine, so he doesn't regret it, but did he make the decision for sensible reasons? What did he say...that it seemed worth considering for future conversations, if this was going to take a while to arrange, but it wasn't his top priority. Because - well, his main argument here is that ten marginal minutes of conversation with Caris, while he's maximally on guard, seems like it should be a small effect next to, well, all the sex.
(Not thinking about the sex. Feelings, box, go. He mentally replays Caris' girl voice, to try to prod his hindbrain into either treating "Carissa" like a different person who isn't his lover, or at least tuning down any feeling of attraction.)
This argument is, of course, assuming that any of his priors about how magic normally works apply here. Kastil made that point and it's...not a terrible one. Implies that interrogating Altarrin is a priority, they can't be entirely sure of anything he tells them but it's got to be evidence.
Gratitudes are exchanged, he doesn't think you need mind control to explain that, saying "thank you" is free and he did owe Caris at least that, he had already decided on it before the conversation, that much should be in his preparatory notes, it could of course be due to earlier mind-control but it's not (much) evidence on whether the additional ten minutes of speaking was dangerous.
He explains where they're at with Altarrin's interrogation. He - was pretty open with it - but he definitely had planned on that in advance, because it's their half of the trade, Caris was going to save Altarrin and give him back rather than stealing him away to hide, and in exchange the Empire wasn't going to make him regret it. He thinks they need that, to go any further, and -
- and he still thinks that's the right choice, because he would give it - hmm - three in four odds, maybe, that this is still salvageable and that trying to salvage it won't destroy the Empire. And he's certain Kastil had contingencies, so - if it goes badly, maybe they just lose but some of that is in worlds where they've as good as lost already, and in some edge-case scenarios, it means, maybe, that the Office of Inquiry has more to work with...
Caris tells him the secret. He prods a bit - he doesn't think any of his questions were stupid - Caris was very cagey but that's understandable, he wasn't expecting to even get this much. He'd hoped the 'how long' question - which Caris claimed not to know but he's very sure that was a misleading not-technically-like and Caris does at least have some bounds on it - might get at ways to check this against the historical record rather than with Altarrin himself. ...Can he just do that, it was implied that Altarrin has 'come back' to the Empire at least once, which begs the question of who he was last time and that...seems like something that someone who knows him well ought to be able to guess at...
Advice on what angles are safer to poke: gods in general, framing the question as 'what convinced you to trust Caris' rather than 'what did you talk about on that first night', it sounds like it should be safe to ask what he knows about Golarion, and magic - in general they would want to give him leeway to answer a question with what he thinks is most relevant and not hazardous to spread.
Or he could ask the lead Mage-Inquisitor to find an interrogator willing to make a redacted report and be executed afterward. He doesn't like it - and it sort of requires not having a Thoughtsenser on hand, they don't have enough Thoughtsensers to start executing them for convenience - but it seems like it might help give a better sense of the constraints, to give alternatives.
(Assuming the Velgarth gods can't get information from dead souls, if they know to look? Caris didn't seem worried about that, though, and honestly reasoning about gods makes his head hurt.)
Caris gave some basically reasonable conditions. They're approximately the same as the proposal the Emperor had already written down – location known only to him, guards loyal only to him, contingencies for if he is himself incapacitated (though he'll need to figure out how to differentiate the case of "assassination by gods" and "assassination by the Office of Inquiry after confirming mind control.") If anything, he was inclined to be more generous and allow Caris some purely defensive magic – not the ability to cast spells, but maybe a Velgarth shield-talisman, which a mage can disable on Altarrin's orders but would offer some protection against a random god-driven earthquake.
Caris wants to run some things by Altarrin. He agreed to allow that. Loosening Altarrin's compulsions to let him speak freely shouldn't be a stretch, you want that for a useful non-Thoughtsensing interrogation anyway, it's way too easy for someone like Altarrin to play the compulsion limiting speech off against the one that's supposed to compel him to answer to his full knowledge. That's probably at least some of why he was so intensely frustrating yesterday. (His confusion probably wasn't directly mind-control, given that he's better today, and confusion due to a high fever and being poisoned predicts recovery overnight more than confusion due to Caris having done something persistent to him when he healed him.)
They agreed to speak in six candlemarks. The Emperor really would rather handle this himself, if nothing else it goes faster when they know their shared context, but he'll run that past the Mage-Inquisitor as well.