Thinking about it really more, it's the banter which really brought out everyone's personalities. You really got to know Auri, Talia, and Iris through what they were talking about in their down time. It wasn't just Auri being hyperactive and Talia's studious habits, but also just how much time Talia and Auri spent talking with each other while Iris hung back, showing that the two had been together for a good amount of time. Carissa and Lyna's interactions were just... argue, then Lyna picks up Carissa with magic. Really, that was their biggest thing.
And while Lyna and Auri giggled over boys (for a minor personality quirk) the new two girls didn't really interact too much.
This is the first that I've heard of executives forcing Lyna and Carissa into the team, although I really wouldn't be surprised. There's also that so much more of S2 was focused on having other things happen (Dark Talia, Dark Auriana, Auri's cousin, Mephisto working with Iris and Carissa) that there wasn't much downtime or many episodes were you necessarily could've worked in the other two. They surely knew they weren't likely to get a 3rd season, and so they wanted to pack a lot of the plot points they could into S2 while they still had the chance.