There's a difference between Harry Kim and Gamora, though. Harry, along with the rest of the Voyager crew, were only a few hours old after the split, so not much time went by and we're basically identical with the exception that one Voyager was badly damaged, the other still in good condition. The Gamora running around now was still a couple of years before we see her in the first Guardians of the Galaxy film. And even then, the events of GOTG and GOTG, Vol. 2 up to IW never happened to her. That's YEARS in difference. Also, Mantis would probably be only to cover the memories from when they met from Ego on forward, not before that.
The best analogy you should be using is the Will and Thomas Riker incident, where they were split in two, but Will continued on with his life and Thomas was stuck for years alone before he was found.
The writers will deliver something very emotionally satisfying is my bet to address it all.
Trust in the Marvel magic true believers. Excelsior!