What happens if one person on 241 redemption doesn't fly back?
 

The missus and I are in India for 2 weeks at the start of January. It might suit me to stay on for 1-2 days on business, thus avoiding making another trip here in early Feb. Assuming I phoned up and cancelled my return flight on the 241 redemption, what would I get back?

a) nothing
b) 25% of the miles spent on the booking
c) nothing

Anyone have any experience of this?

I'd say it's 50/50 between a) and c).

However if you were to OLCI yourself, and call to offload yourself just before the flight departs, I'd say there would be a 50/50 chance that Mrs S might have an empty seat next to her.

There is a good chance that if you cancel your booking then it could cancel both tickets. Better for you to do a no-show - forget about getting the miles back, no chance.