> But it explicitly says it provides the wearer with happiness

It gives you one of the best options for happiness for any decision you face, which is not quite the same. And don't forget it tells you to take it off very first thing.

> Where as a reader are we supposed to gather this is not an internal happiness?

The rest of the story.

> Come to think of it, its not even clear there is a question of agency or like some inverted solipsism as you suggest. Wouldn't the wearer need to be compelled for that to be at stake? Kadmi-nomai can wear it and take it off, right?

It's a problem if you wear and follow it for a very long time. He talked to it for a couple hours.

> Aren't they, from the inside and outside, just listening to magically good advice and getting along great because of it?

Your neocortex is pretty important. You can argue that you think loss of self is unrealistic, but the story is very clear in its judgement. Don't turn a disagreement with the story into "not understanding" the story.