Comment on BTST/ATST( Buy/Acquire Today Sell tomorrow)
Yes correct, check this post.
Hmm.. it can happen at Zerodha as well. You buy shares on an exchange, where we will not know the counterparty. If he defaults, exchange will try getting the share in auction market on 3rd day. If they are not able to get, only then it is settled in cash (highly unlikely).
Yes, you can open an account as NRI with Zerodha.