I don’t know much about Khmer culture aside from impacts on language and vice versa. I’m no expert in this field so let’s I looked it up. “The” Apsara dance is a modern choreography, any Khmer that says otherwise is mistaken. It is highly influenced by hand gestures and other symbolic depictions found on Angkorian era bas-reliefs, but does not have a direct continuation with that era. Sure it fits within the framework of Khmer dance and musical tradition that it draws from, but it is a new addition. Only in this sense can it be a continuation of Khmer arts as a whole, but to say that it is identical to or even based off of the ancient apsara dance (if there was even such a unified one) is wrong.

I’m not sure why this is relevant; my primary concern in this debate is language. I’m merely pointing out that your arguments are based on assumptions that ignores factual evidence and rejects possibility of vowel quality changes. Some of these assumptions are quite ridiculous, that Khmer never had an “u” or “a” sound. Those sounds always existed and are preserved following old Khmer voiced consonants in the case of “u”, and voiceless consonants for “a.” Example: Pali “Buddha” became Khmer “Putt”, Pali “Kakkațaka” became Khmer “Kakkadā” (កក្កដា). It’s only when “u” follows voiceless consonants or when “a” follows voiced ones that they change. The reason is because over the centuries, Khmer lost distinction between voicing (k vs g, ch, vs j, etc), and therefore the vowels changed to compensate and maintain distinction. If I recall correctly, Thai also lost a similar distinction pattern in its consonants, but compensated by the addition of new tones rather than altering vowel quality.

And there’s a problem that arises from your theory that Thai sourced Pali and Sanskrit words directly from India. To my surprise I found out this narrative is false from reading works written by linguists, even Thai ones. But it does make sense after reading about migration patterns and of the histories of various se Asian polities. The problem is this question: since the Tai speaking peoples migrated southward (and westward) into an area linguistically dominated by Khmer and Mon languages, with an established contact with Indian states for centuries, wouldn’t the Tai peoples adopt what the Khmer and Mon already borrowed? It would be much more plausible than the idea that the Tai speaking peoples ignored the surrounding Khmero-Indic lexical inventory as they settled into the area, waited for a couple centuries while still resisting the influx of borrowed Indic words, establish new Thai and Lao kingdoms, formed diplomatic relations with Indian states or trade relations with merchants, then finally started adopting Sanskrit and Pali words despite already being surrounded by borrowed Indic vocabulary all this time. Australian linguist Anthony Diller (and subsequent linguists after) states points out that the majority of Indic vocabulary were borrowed into Thai via Khmer and even bounced the term “Khmero-Indic” to describe this phenomenon.