Mleccha means a foreigner (milakkha in Pali), i.e. non-native people. Such people were a evidently a small minority in Indo-Aryan society, and an exception. In SN56.62, the Buddha is reported as saying “Evameva kho, bhikkhave, appamattakā te sattā ye majjhimesu janapadesu paccājāyanti; atha kho eteva bahutarā sattā ye paccantimesu janapadesu paccājāyanti aviññātāresu milakkhesu” (“People of the unknown foreign/mleccha lands are greater in number than the people of Madhyadeśa janapadas i.e. the Indo-Aryans”). So the understanding of milakkhas/mlecchas is that of foreigners who were foreign to Indian society and not the native races/peoples living in Indo-Aryan (northern & central) India. Mlecchas are not used in co-eval Sanskrit literature to refer to the natives of northern & central India, irrespective of their ethnic/racial origins. They are used as far as I know only for foreign people of the Indo-Aryan borderlands or people who were visitors from abroad. In the śatapatha-brāhmaṇa it is a term used for persians.
The rules in the Arthaśāstra permitting foreigners to own slaves (according to their own socio-cultural norms, or within their own frontier-area settlements) were the exceptions to the rule (of not normally enslaving people born within the Indo-Aryan society).
That does not follow from what Kauṭilya says. There is nothing suggesting that mlecchas means previous native races. They are called foreigners (mlecchas), which implicitly means that they were rather non-native (foreign) to Madhyadeśa.
Dāsa means someone performing indentured service in return for a loan or some other debt until they are able to repay that debt or other liability (usually as a result of their own volition) rather than heriditary or involuntary slavery.
Not evident to me from their speech. They seem to be speaking in fluent Indo-Aryan from what evidence we have from the Pali canon, and there aren’t Tamil loanwords, Dravidian names or other Dravidian expressions visible (perhaps I’m missing something here). Presumably they were not native speakers of Indo-Aryan, but that is at best a guess, and isn’t evidenced either. So I don’t see with what evidence you come to this inescapable conclusion. The names of the dāsas/dāsis quoted above are mostly imaginary people, for ex. the Kaṇha quoted in DN3 is a back-formation from the fictitious name Kaṇhāyana, the Kālī in MN21 is a character that appears in a parable recounted by the Buddha and is equally fictitious. In the mind of the Buddha, or of the narrator of the Pāli canon (in his time and place), those names might have had been appropriate to use for dāsas or dāsīs - but the colour of the skins of most other dāsas/dāsīs, or their races of origin, cannot be extrapolated from the names of these parable characters.
Indo-Aryan India was not seen as the land of the mlecchas, rather the opposite. Mlecchas were the people from abroad who sometimes migrated/visited India in relatively small numbers and were unfamiliar with Indian social norms. Long term settlers such as Dravidians who spoke fluent Indo-Aryan would be expected to know the socio-cultural norms of Indo-Aryan India, and they wouldnt be normally called mlecchas in that case…
Also on a point of linguistic history, Prof. Manfred Mayrhofer’s recent Sanskrit-German dictionary (Etymologisches Wörterbuch des Altindoarischen - you can see a review of it here) indicates that Vedic (approx Pre-Buddhist Sanskrit) is almost completely free of Dravidian and Prakrit vocabulary while new words found in later classical Sanskrit vocabulary (from after the Buddha’s time), contains large numbers of Dravidian, Prakrit, and other foreign vocabulary. That indicates that Sanskrit in India remained relatively free of external influence until circa the start of the Mauryan/Buddhist period – compared to later periods when such influence began to show prominently in Indo-Aryan literature. So there isn’t the hard data to indicate that the majority people of early Indo-Aryan India were not the Indo-Aryans themselves (but rather Dravidians, Munda and other non-IA ethnicities). Dravidian influence on Sanskrit is for the most part after the Buddha’s time.