Need to clarify one thing if theres no reserve keeper in 15, and going into a match in few days time we know that Dhoni is injured, is there a rule not permitting Parthiv to come into the squad in place of Dhoni and keep wickets?
If Dhoni gets injured and is expected to be out of action for 2 matches, then India cannot pick a guy from the 30-man squad.
However, if Dhoni is completely ruled out of the World Cup, then India can replace him with a reserve keeper.
This is where you need a reserve. I think they may carry one around just like they did to Parthiv in South Africa 2003.