In summary, the current status seems to be that D N Nagar Police verified and claim that report was sent to RPO Mumbai but RPO says no such report received from police.
Apart from RPO Mumbai, I think you should also file a
RTI Application with the D N Nagar Police Station to get information on:
1. What date did they receive the papers from Passport Office
2. What date was the person (that is you) called to the Police Station fro verification
3. What was the covering letter number or bundle number under which your verification report was sent to RPO Mumbai.
4. Certified copy of the above mentioned covering letter
Once you have this matter will be crystal clear and you can confront the RPO.
But knowing how the RPO works, you will have to file a simultaneous
RTI application with them based on suggestion of opsharma in the previous post.