You'd have a good case for original routing credit, but your new routing gives you more MQMs than your original routing (even if you don't get the MQMs for OGG-HNL, which is very clearly not allowed by the rules; HA is not a MQM-earning partner). So I agree that the agent at the airport was in error, and the Diamond desk was right.

(This isn't OK; agents shouldn't say stuff that isn't true. Maybe a written complaint would get Delta to throw some small compensation your way based on the incorrect information you were given, and hopefully retrain the mistaken agent.)