I think using Brilliant/Shining and Soul/Heart would be a nice reference to the presence of two characters form two games with those titles. Maybe include Gold/Silver? "Shining Gold Heart", cuz Cynthia is blonde? idk it sounded better on my head.
Down the olivine is my favorite of the bunch though, and for the reasons you described, it kinda works as foreshadowing and I feel it makes sense when compared to the phrase "down the rabbit hole" because of how the protagonists go from storm reports and a random light out at the pokecenter, to stopping a whole terrorist scheme that was connected to everything that happened before.
That name does put more emphasis in the story and less on the girls so that could be a con, but it's the only one that I can see.
Add a scene where the MC goes *down* on Cynthia and the name would be perfect hehehe Edit: NVM the characters literally say that before they do it so now you have no excuse for not using that name.