2. Is there any language that contrasts postalveolar and dental stops? I think that there may be one in Australia, but I don't have enough time to search Wikipedia to find is there any.
Some Australian languages have a four-way contrast between dental, apico-alveolar, palato-alveolar (I suggest that something like this is what you mean by "postalveolar") and retroflex sounds.
If you include retroflex sounds in "postalveolar", you have many languges of India contrasting dental and retroflex stops.
To be specific:
UPSID's online-searchable form lists four natlangs with / t̪ t t̠ ʈ / (that is, in Z-SAMPA, / t_d t t_- t` /); namely, Yolngu (Dhangu), Arrernte (Aranda), Nunggubuyu, and Yanyuwa (Anyula or Yanyula). All four are Australian, and the first two are Pama-Nyangan while the last two are "ungrouped".
Ngiyambaa, another Pama-Nyungan language of Australia, is also listed by UPSID as having / t̪ t t̠ / (that is, in Z-SAMPA, / t_d t t_- /).
Garawa, a Garawan (big surprise)
language of Australia, has / t t̠ ʈ c / (/ t t_- t`c /) -- alveolar, palato-alveolar, retroflex, and palatal -- all phonemically contrasted with each other.
As for voiced
stops. OTOH, the story is different, at least in Frankfurt University's searchable UPSID. None of their languages is recorded as contrasting dental to palato-alveolar, nor retroflex to palato-alveolar, nor palatal to palato-alveolar, in voiced stops. (But 24 languages have alveolar and palatal voiced stops; 12 languages have alveolar and dental voiced stops; and 5 languages have alveolar and palato-alveolar voiced stops. Three languages contrast alveolar with retroflex voiced stops.)
On the ZBB, user "2+3 Clusivity" recommended Dravidian languages to me, in particular Malayalam and Toda. I imagine those must be some of the Indian languages Xing was referring to. Their Wikipedia articles show what he was talking about.