Something Sanki said in the feet washing scene just came to mind, that Raghav is the kind to leave someone to die. Perhaps he hit Mandaar and left him bleeding on the road, either because he was drunk or because he was chasing Ved and didn't want to let him to. It might also be that Mandaar was already on the road hurt by someone else and Raghav's car hit him again or Mandaar had suddenly come/been thrown in front of the car by someone else, too late for Raghav to break.
He probably never went to check back on Mandaar or when he did, they had taken his body away, so he never knew what happened to him eventually. Now knowing that he was Pallavi's first husband who had died on that night might have helped him connect the dots and realize Mandaar died because he wasn't treated on time or something. This might be why Raghav feels guilty now because of the connection to Pallavi, otherwise this isn't the kind of incident he'd forget about and not feel guilty if he had done this deliberately, based on what we've seen of him so far. He's so guilty about his family members' deaths even though he wasn't at fault so imagine if he had intentionally killed Mandaar. He might have kept the ring since he found it on the road when he came to look for Mandaar later on. This is why he remembers the date so clearly, as it's engraved on the ring.
BWA might be the one who brings back a fake Mandaar based on what she said today or she's the one who's kept Mandaar hidden all this time for some reason. Who knows? I want BWA away from this whole matter but I don't trust her. When Milind told her about Raghav and Pallavi's relationship going in the right direction, she didn't like that at all.