I'm going with the same assumption - Katie has interest in both of them, but we don't know why and we're makign our own conjectures. She could have interest in either of them for separate/different reasons. We also can assume (reasonably) that Nami met a Zane while Summer was still around (to receive the earring) which means Noji isn't necessarily hiding them well, if that's what she's trying to do. It's also easy to assume that Katie spoke with Nami for the length of time Nika spoke with Noji, but it's off screen, so you're right, we don't know how long their interaction was. She might've just said hi the same way to both of them.
About eye colours - Cerill looks comparablely close to Bella and Nika, but the lighting of the scene she shares with them makes it hard (almost impossible) to tell. Claire is yellow/brown (similar to Susan) in the bookclub, and Penelope has green with a slight yellow tinge (again depending on the lighting in the render). I'm pretty sure can assume Amber is not a Zane given her age. Given Vic is more similar to Claire rather than Maja, and even Susan and Bella have similar depending on the lighting.
After all, in the bookclub scene Katie is clearly blue, and so is Nami, and Katie and Emilia are 100% identical in colour.
Below are just a few renders that kinda show to me I'm going to stay out of eye comparison debates - Ocean's light sources are never neutral and change everything. Below we have a case for the maternal side of the Zane's being blue (Scarlet is also blue, and as you corrected me Kat is also blue), while Leia and Dylan are the odd ones then. Also, Vic is closer to Nika than to Maja, Summer and Nami might as well be twins...