I think that letter shown in the promo will contain some secret that can be understood by Naina only.Bcoz he had asked Raj to give the tape to Naina.
Moreover i guess he trusts Naina more than Raj-Abhi becoz if he had trusted them more,he'd have mentioned abt it or something related to it to Raj atleast,who was very close to Navin.RAJ COULD NOT SOLVE THE CASE IN 4 YEARS, BEING A CAPTAIN.
Else Navin'd have wanted to bring RN together for this mission.