This needn't mean that "the true terminal value" is pleasure or other positive emotion

True, it wouldn't mean that pleasure was the actual terminal value, and the fact that many people reject wire-heading is evidence that pleasure is indeed not a terminal value for those people.

However, what role could "happiness" or feelings of well-being play, if not as true terminal values, if it's in response to those feelings that people change (what they thought were) their terminal values?