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).
