Yes, śak (dhātu/verb-root) + raK (pratyaya/affix) = śakra (one who is powerful, competent or able). ‘Able’ is probably not the meaning that makes the most sense for Indra, ‘Powerful’ should make better sense.
The Abhidhānappadīpikāṭīkā gives this definition: “asure jetuṃ sakkuṇātīti sakko”
I think here purandara is being used as the adjective for śakra - as adjective(s) normally precede the substantive they seek to describe.
Since there are not regular phonetic/sound shifts that show etymological relations between purandara (Skt) and purindada (Pali), I looked at the kharoṣṭhi script forms - and sure enough they look close (attested shapes in inscriptions show the syllable ‘ra’ being written like ‘da’ by some scribes historically). Thus purandara > purindada is not a dialectal change but an orthographic change.
The first word below is purandara (Sanskrit), the second is purindada (canonical pali).
Even in Latin characters they look nearly identical. The idea that there is influence from manuscripts in Pāli is of course undisputed. The idea that it goes back to the beginning really has no basis as I see it.
Just imagine: the Buddha spoke some MIA language which was compiled and memorized, then passed down for a few centuries. At a certain point, it was written down, and ever since then there have been word forms which seem to be related not to phonological shifts but shifts in the manuscript. In fact, this is one of the arguments Karpik makes in his article contra Levman — the idea of some prakrit lingua franca “behind pali” due to word form variation can often be reduced to the manuscripts and copying mistakes, not phonological change. And he uses this to maintain that something close to Pāli and the Ashokan edicts was spoken and memorized, not that it is merely some orthographic convention when people are perfectly capable of writing proper Sanskrit.
Pali canonical texts have lexical variations for some words - fact
Some of them can be explained by looking at the same words written in Kharoṣṭhi script - this is what I am showing here, here and here apart from the above post. They look similar enough to be misread, and such spelling mistakes have actually happened in historical inscriptions written in Kharoṣṭhi. Even Ashoka mentions the existence of such spelling mistakes in his inscriptions in his Rock Edict 14 (“dipikarasa va aparadhena” i.e. “mistakes committed by the writers”)
Kharoṣṭhi spelling mistakes affecting vocabulary in extant Pali manuscripts cannot have happened anytime in the last 2000 years, it would really have to go back to the very first Pali manuscripts that were ever written - only they could have relied on pre-existing Kharoṣṭhi manuscript sources for their spellings.
So your disagreement is incomprehensible.
There is no need to imagine those things - as they are not an answer to the issue of lexical variations attested in Pali canonical texts. The Pali canon was not written down from any oral tradition but from pre-existing Kharoṣṭhi manuscript sources. The evidences I am presenting explain exactly how the lexical variations in Pali words that I have mentioned would have arisen as a result of confusions arising from using the Kharoṣṭhi texts as canonical sources for the Pali canon.
Such lexical variations or confusions do not exist in Brāhmī script (in which Pali was originally written) - the shapes of letters in Brāhmī are very different from the shapes of letters in Kharoṣṭhi - so your speculation that they arose somehere along the line rather than at the beginning of the manuscript tradition doesn’t appear logical - as Kharoṣṭhi script went out of use around 250 CE (i.e. it was not used thereafter) and orthographic confusions in Brāhmī script (canonical pāli’s earliest native script) from a later date would not explain these specific lexical variants.
Each script has its own peculiarities (Kharoṣṭhī is written from right to left, while Brāhmi is written from left to right) - and mistakes that can happen in one script do not normally happen in other scripts (or in oral traditions), and vice versa. For example, the letter u & v can possibly be confused in the roman script - but in Brāhmī, it would probably never happen - as the shapes 𑀉 (u) & 𑀯𑁆 (v) are entirely different. So similar looking lexical variants in Kharoṣṭhi would not necessarily look similar in Brāhmī - so we can reasonably estimate which script those variants emerged from (and in which century - as shapes of letters in each script evolves across each century of use).
I think you might be overlooking the more specific and older parallel, exemplified by the Greek Antaeus or Norse Baldr, of a character being protected under circumstances that in their initial phrasing seem universal, until their opponent finds the one situation that wasn’t covered by the invulnerability. This fits within your idea of the victory of “brains” but is narrower, seemingly addressing what we today would call “out of the box” thinking.
Also, these stories, being present from end to end of the Indo European cultural sphere, and in the basic mythological strata, likely have a common origin or parallel in Porto-indo-European myth, whereas Odysseus is a literary character who comes from a later strata more in line with the elaborated version of the Ramayana (even if those have no elements of common origin / exchange, both were composed in societies that had reached a different stage of development than PIE nomads, e.g. genuine interstate warfare, and are preserved in the genre of epic poetry).
