I think I wasn't clear in my explanation of (8), so just to clarify further:
I meant that ストレス was an emotion, not 取れて. This being said,
ストレス is a noun here, not an adjective. I would have said the reason is: When predicates are transitive adjectives or stative transitive verbs, the elements which corresponds to the direct object in English are marked by "ga".
But... ストレス is a noun and 取れる is intransitive. So I'd say が as the subject of the sentence.
*
取れる is intransitive which is why を is out of the question.