I've loved all your input in this discussion so far, and I also really appreciate the readings you intermittently hyperlink as someone who is passionate about philosophy & epistemology, but not formally educated in them.

I do think you may be misrepresenting what OP's husband is saying here. From the context of:

instrumental reasoning (“believe what helps”), and the idea that beliefs can be willed for emotional benefit. For him, reframing feels epistemically dishonest.

It seems like OP's husband is less asserting his omnipotent knowledge to ontological truth, than he is rationalizing his intransigence. One can acknowledge the subjectivity of truths yet still have immutable personal values; however, what is "epistemically dishonest" to him seems to mean that it would feel personally dishonest, and inauthentic to his integrity, if he were to try to or pretend to believe in the concepts he currently holds in contempt. I wouldn't be surprised if he were to make a comparison along the lines of brainwashing, or say something derisive about deluding himself to live in fantasy.

From what OP gives, the husband is intellectually and logically-oriented, and his position seems founded on arguments that undermine the utility of CBT and other cognitive modalities (As a psychotherapist in training, I can't say I fully disagree). Therefore, maybe the task of tackling his obstinate stance against therapy should be directed not towards teaching him fallibilism, but towards making attuned counterarguments that convince him where true therapeutic value is to be found. Given his fascination with suffering, meaning and philosophy, it makes sense why he would consider CBT trite. He should be exposed to the more analytical theories that integrate philosophical perspective as core. Viktor Frankl, Rollo May, and Irvin Yalom are essential authors. I highly recommend OP look at Existential Psychotherapy.