I was reading the Cthulhu Dark rules again, and I noticed something that wasn’t specified.
The section on rerolling says, in part:
If you included your Insanity die in the roll and you’re not happy with the result, you may reroll (all the dice). If you didn’t include your Insanity die before, you may add it now and reroll.
Afterwards, look at the new result. As before, the highest die shows how well you do.
It does not say anything about what happens if the highest die after rerolling is lower than the highest die before the reroll. Do you still use the new roll if the highest die is lower than the higher die before rerolling?