Learn English – Relatively recent history of bloodbath

The Oxford English Dictionary doesn't have doublespeak as a separate entry; I can only make an educated guess. The word double-talk already existed in 1938, according to the OED, and was originally American. But apparently it meant simply "deliberate gibberish" then, i.e. not merely ambiguous language, but nonsense talk that listeners were supposed to know meant nothing, uttered to comical or artistic effect.

1938 New York Panorama (Amer. Guide Ser.) vi. 156 Of late a humorously conceived system of language corruption called double talk ... has made itself felt. Ibid. 157 Double talk is created by mixing plausible-sounding gibberish into ordinary conversation, the speaker keeping a straight face or dead pan and enunciating casually or off the cuff.

1941 Time 16 June 61/1 Thirteen recorded versions of this pandemic double-talk ballad are available.

1945 H. I. Phillips Private Purkey's Private Peace xxii. 129 We got the right slant on bullies, greaseballs, double talkers, supermen, and dopes.

1948 Auden Age of Anxiety vi. 125 And all species of space respond in our own Contradictory dialect, the double talk Of ambiguous bodies.

After Orwell's famous novel 1984, published in 1948/1949, double-talk apparently acquired the (additional?) meaning "deceiving language", i.e. language that is deliberately ambiguous, or language that appears to mean one thing but in fact means another.

1950 Amer. Speech XXV. 190 Back in Tsarist times Lenin and his associates inaugurated this double-talk and double-writing—especially the latter—in order to deceive the Tsarist censors and police.

1952 C. Day Lewis tr. Virgil's Aeneid ix. 203 You'll find no Atridae here, no double-talking Ulysses.

This is probably based on Orwell's newspeak and doublethink. His novel was (partly) a critique of the practice and propaganda of the totalitarian regimes of his time, Fascist Germany and Stalinist Russia.

Newspeak is new language invented by politicians as propaganda, in order to influence people's thinking by changing their language (one of the principal mechanisms studied by Postmodernist philosophers and sociologists). The word is invented by politicians in the novel and is an instance of itself. By drastically reducing vocabulary, and making new, simpler words from the remainder, they hoped to control the people more effectively. Many modern euphemisms could be called, and are often called, newspeak, like "differently abled" and "Secretary of Defence".

1949 ‘G. Orwell’ Nineteen Eighty-Four i. 51 Syme was a philologist, a specialist in Newspeak. Indeed, he was one of the enormous team of experts now engaged in compiling the Eleventh Edition of the Newspeak Dictionary. Ibid. ii. 133 Do you know the Newspeak word goodthinkful? Ibid. App. 299 Newspeak was the official language of Oceania and had been devised to meet the ideological needs of Ingsoc, or English Socialism. In the year 1984 there was not as yet anyone who used Newspeak as his sole means of communication, either in speech or writing.

Doublethink is somewhat related to what modern psychology calls cognitive dissonance: first accepting one fact as true, then another, contradictory fact—without critical self-assessment, which would ordinarily result in elimination of one fact or the other. Orwell's politicians try to effect doublethink by means of using and propagating ambiguous language.

1949 ‘G. Orwell’ Nineteen Eighty-Four i. iii. 37 His mind slid away into the labyrinthine world of doublethink. To know and not to know, to be conscious of complete truthfulness while telling carefully constructed lies, to hold simultaneously two opinions which cancelled out, knowing them to be contradictory and believing in both of them, to use logic against logic, to repudiate morality while laying claim to it, to believe that democracy was impossible and that the Party was the guardian of democracy.

Considering the change in meaning of double-talk soon after the publication of Orwell's novel, and the emergence of doublespeak to mean the same a few years later, your sources (reasonably) assume that the word was deliberately coined as a (semi-)portmanteau of Orwell's newspeak and doublethink. The OED has 1957 as its earliest quotation:

1957 ‘M. Buttle’ Sweeniad ii. 55 In the literary weeklies, the languages of criticism and theology have become one and book reviews all sound like sermons written in the most holy ‘*Double-Speak’.

1961 W. Kaufmann in G. E. Myers Self, Relig. & Metaphysics 99 The theologians have a way of redefining terms in rather odd ways, and frequently engage in something best called double-speak: their utterances are designed to communicate contradictory views to different listeners and readers.

The asterisk and quotation marks in the first quote might indicate that the word was then quite new and explained in a footnote.

The "open-arse" (also: enter "open-ærs") entry's first reference in the NED is: "c. 1000 AElfric Gloss. in Wr.-Wülker 137/36 Mespila^1/1a, openaers." This source contains no context as this is a lexicon geared at scholars (see document intro.). A note showcases the reaction of Wr.-Wülker to the word, much later, in 1884:

It is rather singular that we should find this not very delicate name for the medlar at so early a period. It is found in the MSS.[manuscripts] of the fifteenth century and is a word sufficiently familiar to the readers of the popular literature of the sixteenth and seventeenth centuries.

[ Archbishop Alfric's Vocabulary of the tenth century, 137/36, note 5 ]

The NED (1888) mentions for "arse, sb.", "Obs. in polite use.". This should mean it was not always impolite. The first meaning listed refers to this being about an animal (also, ex. of the figurative meaning: "In Cheshire the stalk-end of a potato [is called] the 'arse-end of a 'tater'".) And the entry also refers to AElfric 44/2 where indeed you have Nates (Latin for buttocks), to "ears-lyre"(OE); but there is no note from the author this time over (maybe because it's not spelled "arse"? yet open-ears to mean open-arse might have been). Ass in its slang version as a dialectal variation of arse is recent (1860, popular 1930s). Considering this, a reaction seems unwarranted imho: it's about something descriptive as in the context of farming/hunting; I cannot find any alternative to Mespila or open-ærs/arse/ears in the material presented.

Blet comes from the French blettir (from blet adj., which is from an older form of blesser, but related to bruising as opposed to wounding: "Mil. XIes. blecier « meurtrir (des olives, des fruits, pour les faire mûrir) » (Raschi, Gloses dans Levy Trésor)" - as in making ripe olives, fruits by this action of bruising.) Something we casually do with our hands when some fruit is not ripe enough. Or we just wait for it to happen, as in this case systematically with some prior peeling. This answer presents the "special alteration" coined by M. Lindley for which this "blet" now stands for in context.² Rot has a different etymology with for instance a German cognate rößen "to steep flax" ("letting raw flax steep under water for several days to break down the lignine in the stems[...]"). It may have no impact on contemporary use, but the first entry in the NED for the verb to rot relates to "animal substances" in the context of natural decomposition. In comparison blet is mostly a special form of decay for fruits.

_{1. More generally, the category to which the medlar belongs is interesting because other members enjoy in some cases a very rich naming tradition; such is the case with the amalanchier (also): shadbush, shadwood or shadblow, serviceberry or sarvisberry, or just sarvis, wild pear, juneberry, saskatoon, sugarplum or wild-plum, and chuckley pear. The fruits can be 50% smaller, and sometimes more of the berry type but they're related.}

_{1a. See Plinius Natural History (AD 77–79) with translation. The TLFi etymology entry of the French word for the fruit (nèfle) refers to Plinius possibly meaning "white thorn" from the Greek "μεσπι ́ λη". This brings meaning to the medlar wikipedia entry's reference: "When the genus Mespilus is included in the genus Crataegus, the correct name for this species is Crataegus germanica Kuntze." The Crataegus is basically hawthorn/whitethorn etc. which are all, like the medlar, part of the rose family; the hawthorn is graft compatible with the medlar. In that context the haw refers to the fruit; often compared with the medlar.}

_{2. The reference to the "special alteration" is from the same work; see NED entry, which also has: "The decomposition... of the pericarp[the "flesh", see this] begins with fermentation, and, after having passed through the intermediate stage of bletting[...], ends in the total obliteration of the cellular structure." (1864, Reader, 21 May 663). See also following entry with blet sb. where there is indication that there are no external changes appearing with sleepiness i.e. blet, as opposed to spots etc.}