Regarding puñña and kusala, I have said my (native) opinions about them as I have a native sense of what they mean, you can follow whatever else you find appropriate. Sure, as you say not all meanings are appropriate in every context, but you will still need to make up your mind on which meaning fits which context.
Yes - they are different - no disagreements there - however most are surface-level differences. Also, of all languages that you could call similar to Pali, Classical Sanskrit (or late-vedic) is probably the nearest. Early-Vedic (which is normally called Vedic), on the other hand, is very far from Pali.
Classical Sanskrit too had vedic archaisms at that time, and it’s not just Pali that has them - and therefore there is no philological need to invent a parallel evolution for Pali from Vedic (rather than linearly from Sanskrit), or go even beyond Vedic into Proto-Indo-Iranian (to explain what amounts to a few, possibly Iranic, loanwords).
As I have said elsewhere – there is no hard and fast rule demarcating classical sanskrit from vedic sanskrit, and vedic grammatical and other archaisms did continue in classical sanskrit even after Pāṇini (for example Patañjali, in his 2nd century BCE commentary on Pāṇini’s grammar, the Vyākaraṇa-Mahābhāṣya, written in classical sanskrit, uses the vedic ‘tavai’ infinitive affix – “tasmād brāhmaṇena na mlecchitavai nāpabhāṣitavai”). Other classical sanskrit texts from the period do have such archaisms too. So Pali is not uniquely non-derivable from classical sanskrit in that respect - it is a popular misconception.
I’ve said about this here - and I restate:
I have read such claims and I find them greatly exaggerated. Those words that are cited as evolved from variant pre-Pali dialects are less than 1% (or even 0.1%) of the Pali vocabulary - and they may very well be Iranic (Old-Persian / Median / Avestan / Scythian) loanwords - if at all they are of Indo-European origin. I dont even think such words exist in the later Pali texts or commentaries. So they are very likely Iranic loanwords from a time when the Achaemenid Empire ruled parts of North-Western India (which I believe was the homeland of Canonical Pali & Gandhari) from Persia. The period of Achaemenid suzerainty was when the Buddha lived and died.
They vast majority (circa 99%) of Pali word forms are phonetically simplified variants of classical sanskrit words and are therefore capable of being etymologized from classical or vedic sanskrit. Can any scholar prove (or has any scholar proven already) that this is not so?
Besides there is some clear evidence about the Buddha’s spoken language from the Ud 5.6 – a statement that Soṇa Kuṭikaṇṇa recited (abhaṇi) the aṭṭhakavagga “with svaras” (sarena) to the Buddha, and at the end of the recital with svaras (sara-bhañña-pariyosāne), the buddha lavished praises on him for his clear and correct enunciation. But what exactly are the svaras? I take them to be the vedic svaras (the tone accents) - he recited the verses with the tone accents. The accents (svaras) exist in classical sanskrit (where they are optional) as well as vedic (where they are compulsory), so what other language other than Old-Indo-Aryan (Classical Sanskrit or Vedic) could Soṇa Kuṭikaṇṇa have recited the aṭṭhakavagga to the Buddha in? The Vedic svaras (tone accents) are inherited from the Proto-Indo-European, see Proto-Indo-European accent - Wikipedia and Vedic accent - Wikipedia . For more about this topic, see my posts here
It’s already been discussed. I don’t think he did. In any case, a lot of Buddhists thereafter defied him by doing exactly that. Maybe the Theravādins alone persisted with that notion.
Many of them are superficial opinions though. Very few are really thought out in depth.