Based on your quick research, you came to this conclusion: “With all the examples presented here, my observation and readings led me to conclude that sequence of tenses only operates in indirect structure, complex sentences which uses subordinating conjunctions or adverbial clauses, and in paragraph level but with exceptions.”
This is fine as it goes, but I’m getting this feeling that what you are attempting is to formulate some sort of “Universal Theory of Gravitation” on something so disparate in properties, attributes, and functionality as the elements of the English language, which of course includes such aspects as vocabulary, grammar, syntax, semantics, sentence structure, idioms, and the peculiarities of usage. Such an enterprise could lead you to a wild-goose chase of particular sentence constructions that will fit your theory that “noun clauses grammatically functioning as subjects in a sentence should follow the sequence-of-tenses rule,” when the preponderance of evidence militates against that possibility by virtue of the nature of language itself. As I pointed out in a previous posting, it may be possible to find particular sentence constructions that appear to fit that theory of yours by simple accident or happenstance, but coming across just a single exception actually would be enough to invalidate your theory.
Let me just pick at random from the examples you presented of sentences with a noun clause grammatically functioning as their subject:
(1) “What I
had for breakfast
gave me heartburn.” The sequence-of-tenses rule doesn’t come to play at all in this complex sentence construction. The noun clause “what I had for breakfast” is something the speaker had eaten in the indefinite past, and the tense of the verb that follows it could be anything at all and will not conform to the sequence-of-tense rule that you have in mind: “What I
had for breakfast
gives me heartburn.” “What I
had for breakfast
has been giving me heartburn.” “What I
had for breakfast
certainly will give me heartburn.” “What I
had for breakfast
could give me heartburn.” “What I
had for breakfast
might give me heartburn.”
(2) “The girl who
is singing the song right now
had an affair with the CEO.” The sequence-of-tenses rule likewise doesn’t come to play at all in this complex sentence construction. The relative clause “who is singing the song right now” modifies the subject “girl” and, together, they form a subject that can be followed by a verb of any tense or modality whatsoever: “The girl who
is singing the song right now
has an affair with the CEO.” “The girl who
is singing the song right now
will have an affair with the CEO.” “The girl who
is singing the song right now
might have an affair with the CEO.”
(3) “The highway department spokesman
said that the new highway, which
connects three southeast Kansas cities to the turnpike,
was closed in three places by flooding.” This is a legitimate, authentic reported speech that uses the past tense for the reporting verb (“said”) and where the operative verb of the utterance took one tense back (“was closed”) following the normal sequence-of-tenses rule. The modifying phrase “which connects three southeast Kansas cities to the turnpike” is in the present tense because of
the “eternal truth” present-tense rule or permanent condition rule. Keep in mind that in direct speech, that reported sentence would take this form: “The new highway, which
connects three southeast Kansas cities to the turnpike,
is (or “has been”) closed in three places by flooding.”
(4) “Salmon, which
live in salt water but
spawn in fresh water,
flourished in the Northwest.” The sequence-of-tenses rule doesn’t come to play at all in this complex sentence construction that clearly has no reporting verb. It’s actually a simple sentence, “Salmon flourished in the Northwest,” with the subject “salmon” modified by the relative clause “which live in salt water but spawn in fresh water” as appositive. Indeed, this basic sentence is in the classic subject-verb-predicate construction where the operative verb can take any tense or modality whatsoever: “Salmon
flourishes in the Northwest.” “Salmon
has flourished in the Northwest.” “Salmon
had flourished in the Northwest.” “Salmon
will flourish in the Northwest.” “Salmon
could flourish in the Northwest.” “Salmon
might flourish in the Northwest.” The tense or modality ultimately depends on what the writer or speaker has said or wants to say. You can insert the appositive relative clause in any of them without causing any problem or sequence-of-tenses complication at all.
I could go on and on evaluating so many other examples, including the latest batch that you have provided, but I think the analysis above will suffice to convince you that it isn't advisable or warranted to further pursue your theory that “noun clauses grammatically functioning as subjects in a sentence should follow the sequence-of-tenses rule.”