When oversold, airlines in India are very rigid about checkin cutoff times, they always get a few that way

You are offered to be re-booked for free and thats about it. 9W/AI will put you up in a decent contracted hotel and feed you there. The DGCA has regulations in place that are similar to EU261, the IDB compensation is peanuts though when converted to €/$.

Non-EU carrier departing to the EU doesn't have EU261 protection.

The carrier rebooking you is supposed to check if the carrier transporting has positive space before rebooking. If the accepting carrier has no proof it accepted the endorsement, it can deny checkin and send you back to the original carrier.