Luckily, they have to do the part with the devil outside the Forbiddance since you can't summon an outsider into one; that means there's an ironclad excuse for Keltham not to be present for it.  Carissa is hoping that the reason for the instruction that she is not to sell her soul this day is in fact that she's meant to do this, as part of seducing Keltham to Evil. If not, though, and they still won't buy her soul, she wants to draw up a very real contract giving Keltham a very real option on her soul, which avoids specifying that it is presently owned by Hell, which she and he will sign. And then get permanent Arcane Sight and claim the Hell part of the contract went off perfectly, or if they don't want to give her permanent Arcane Sight, claim that it worked but the option makes it much less valuable so she doesn't get the perks.

Unless to the High Priestess's awareness this is a stupid plan, in which case she can just tell them that options like that don't exist to her knowledge.

Luckily, they have to do the part with the devil outside the Forbiddance since you can't summon an outsider into one; that means there's an ironclad excuse for Keltham not to be present for it.  Carissa is hoping that the reason for the instruction that she is not to sell her soul this day is in fact that she's meant to do this, as part of seducing Keltham to Evil. If not, though, and they still won't buy her soul, she wants to draw up a very real contract giving Keltham a very real option on her soul, which avoids specifying that it is presently owned by Hell, which she and he will sign. And then claim that it worked but the option makes it much less valuable so she doesn't get the perks, to explain the lack of Permanent Arcane Sight.

Unless to the High Priestess's awareness this is a stupid plan, in which case she can just tell them that options like that don't exist to her knowledge.