Sekha: It is very improbable that insulting the person to be taught, who is already feeling negative, would do any good.
Who said anything about ‘feeling negative’? I presented a scenario where the audience is ripe for conversion or awakening, but impeded by conceit of the kind that makes one intractable and contemptuous towards the Teacher. If this conceit is grounded in the notion of the intrinsic spiritual superiority of one’s social class, then the way to remove it is by drawing attention to the inconsistency between the sacred lore (with its ascetic values, etc.) to which the audience declares allegiance and the audience’s carnal and cyrenaic manner of living. One might compare it to Gandhi’s use of New Testament rhetoric and the British self-image of being “the more civilized” sort of imperialist to expose British imperial hypocrisy, thereby shaming the British into bending themselves to his wishes.
Well, the fact that the first part of the "device" at stake may not be in accordance with the Dhamma proves that it is not the word of the Buddha and that therefore it must be a late addition.
What a singular criterion you propose. Do you mean that your hypothesized original Sutta Piṭaka comprised only the words of sages and that all the adhammic utterances attributed to, say, Devadatta, Sunakkhatta, Ariṭṭha the Vulture-Basher, Tissā the Fat, etc., were added later?
Now if that happens within suttas otherwise considered as genuine, there is no reason not to think that some short suttas may have been completely made up.
“Some short suttas”? For practitioners of the Sekha Stratification Method I think consistency would actually entail the excision of most of the Devatā-saṃyutta and many dozens of suttas —short, medium-sized and long— elsewhere in the Canon. Any sutta that takes the form of a deva, yakkha, brahmā etc., saying something silly in verse and the Buddha replying with something sensible, or a deva saying something sensible but concerned with a lower good, and the Buddha replying by pointing to a higher good, will have to be given its marching orders.
Besides, what we have here is not merely "some mundane commonplace" nor a "pedestrian utterance"; it's a bombshell !
The noble warrior is the best among people
When judging by clan.
Then we must be of differing sensibilities, for to me it’s no bombshell at all. As the scion of a Royal Air Force family I’ve never entertained any doubts that we kṣatriyas are the “seṭṭho janetasmiṃ”.
As to “mundane commonplace” and “pedestrian utterance”, these are fair paraphrases of the Majjhima Ṭīkā’s gloss on “seṭṭho janetasmiṃ”. It states that the kṣatriya class’s superiority is but a mundane convention whose scope of application is limited to the human realm and doesn’t extend to the worlds of devas and brahmās (’khattiyo seṭṭho’ ti lokasamaññāpi manussalokeyeva, na devakāye brahmakāye vāti dassetuṃ
I note that in the suttas the Brahmā deity Sanaṅkumāra (= Sanatkumāra of the Chāndogya Upaniṣad  and perhaps Sir Galahad of Arthurian tradition ) is always saying “khattiyo seṭṭho janetasmiṃ”
; in fact it seems to have been a bit of an obsession with him. He says it not only in the Sekhasutta, but elsewhere in all four Nikāyas: the Ambaṭṭha and Aggaññasuttas of the Dīgha, the Sanaṅkumārasutta of the Saṃyutta, and the Moranivāpasutta of the Aṅguttara. And the Buddha himself says it in the Mahākappinasutta of the Saṃyutta.
The brahmin caste has always been considered has the highest caste.
By brahmins, certainly. A four-tiered class system with brahmins on the top was how the brahmins thought a society ought to be
arranged —their revealed scriptures said so— but it’s not evident that brahmins contemporaneous with the Buddha were enjoying much success in implementing such a scheme. One should be wary of uncritically assuming that the sort of social conditions that obtained in post-Manusmṛti India, when brahmins were very often to be found ruling the roost, necessarily obtained in earlier times. 
Is it really reasonable to believe that a Brahma would appear in the human world for nothing more than making some controversial statement about a petty human dispute?
But Sanaṅkumāra doesn’t just do that. He repeatedly descends to earth to let folk know that…
“... a person consummate
in clear-knowing & conduct,
is the best of beings
human & divine.”
and the preceding part of his speech is just spice.
 Where too we find a pretty glowing endorsement of kṣatriyas:
The king commanded him: “Stay with me for a long time.” Then he said to him: “As to what you have told me, O Gautama, this knowledge did not reach any brahmin before you. Thus it was to the kshatriya alone, among all the people, that the teaching of this knowledge belonged.” Then he began to teach him…
 Suggested by Th. Rhys Davids in Dialogues of the Buddha
 Some helpful general remarks on ‘caste’ from Paul Williams:
Scholars tend to think of Brahmanism at the time of the Buddha not in terms of the Indian actuality of caste (jāti) as it has developed over many, many centuries, but rather in terms of the Brahmanic ideology of class (varṇa). Note this distinction carefully, because confusion between caste and class seems to be almost normal in works on Indian religions. Classical Brahmanic texts dating from Vedic times and beyond refer to society divided into the four classes (varṇas) of brahmins (brāhmaṇas), warriors/rulers (kṣatriyas), generators of wealth (vaiśyas), and the rest (‘servants’, śūdras ). This division is by birth, it is a division of purity, and it is strictly hierarchical. Each preceding class is purer and therefore superior to the following. Thus the preceding class has a higher social status than the following, quite regardless of any wealth one might have.
Within this system there is no correlation between wealth or power and social status. Status is determined by relative purity. It is not given by wealth, power or, as such, behaviour or insight. Members of the first three classes are referred to as ‘twice-born’ (dvija), and they are entitled and expected to enter into the world of Vedic religious duties, for most of their lives as married householders. This involves keeping alight the domestic sacrificial fire and engaging particularly in the duty to sacrifice, each in the appropriate and distinctive way determined by relative position (relative purity) in the social hierarchy. Nearly everyone can be fitted somewhere into one or other of these classes. Which class one is a member of determines (according to the Brahmanic lawbooks) a whole range of social behaviour from who one can eat with to which sort of wood is used in making one’s staff, or which sacrifices have to be carried out, by whom, and at what age.
Over the years Indian social actuality going back many centuries has seen not just four but hundreds of castes (jātis) and subcastes. If we try and relate class to caste, varṇa to jāti, class is classical Brahmanic ideology while caste is historical and modern actuality . They are different. The varṇa system is what the Brahmanic authors wanted to see, and to the extent that brahmins were the dominant group in society the varṇa ideology provided a template for what they sought to realise.
The jātis represent the actual system of Indian social division within relatively recent historical time. It is important to preserve the terminological separation of the two, and not to confuse them. At the time of the Buddha there was the ideology of varṇa, that formed part of the ideology of brahmins, the dominant group in much of North Indian society. No doubt there was within that area also some form of social division influenced to a greater or lesser degree by the ideology of varṇa . But the extent to which the varṇa ideology influenced the actual social divisions in the region from which the Buddha came, a fringe area in the Himalayan foothills, is still very unclear.
The Buddha was critical of the intrinsic supremacy of the brahmins, and with it the ideology of varṇa. But it would be misleading from this to infer, as some modern writers do, that the Buddha was ‘anti-caste’. First, a criticism of the varṇa system is not in itself a comment on jāti, caste, although it could be transposed to the ideology that nevertheless underlies caste. For his part the Buddha spoke of the true brahmin as one who had spiritual insight and behaves accordingly (see the famous Dhammapada Ch. 26). In this sense the Buddha affirmed a hierarchy not of birth but of spiritual maturity. It is not obvious that the Buddha would have any comment to make about a brahmin who is also spiritually mature (understood in the Buddha’s sense). The Buddha was not offering social reform. And this is what one would expect. The Buddha was himself a renouncer of society.
(Buddhist Thought, pp. 13-15)