Hmm, I both agree and disagree with you there. Kind of 50/50 for me.
Personally I never had trouble telling the trio apart. The only one who I won't argue is not a strong character purely based on interaction, is Eve, so I'll have to agree with you that much.
I also agree that if you judge based on how much you can learn about their backstory without the diary, then they're very poorly handled.
As I mentioned somewhere above, Eve definitely gets hit HARD when you take out the diary scene. I remember that prior to reading the diary, what made Eve interesting to me were her interactions with Zero, as they are plenty and make up the most part of their dialogue with each other.
They're hands-down my favourite character dynamic in the game.
But if you take away that one little comment Lumi makes during the first PULSEgrowth scene, there's no indication whatsoever that she and Eve are any more connected than either of them are to Zero.
And that's a problem, because introducing the entirety of their relationship via the diary makes it a case of something being told, but not shown.
Eve doesn't have to break character to give us an impression of her caring. Maybe have her try to give a panicked Lumi a reassuring (but still calculated) comment every once in a while. Make it more obvious when she's interrupting Zero if he's being rude to her.
And for the love of Arceus, I'm still bothered by the moment during their intro where Eve interrupts Lumi.
Similar to how you said it's easy to establish Hardy's character in his introduction, ZEL's intro worked just as fine for me to establish Zero and Lumi. Zero immediately shows himself as impatient and bratty, whereas Lumi comes across as friendly and meek. It's one of the reasons I liked them from the first scene on.
The big problem with them, IMO, is not about giving us an impression of their personalities, it's that unlike other characters whose diaries you can acess (Rini, Julia, Tania, Amy), you have no way of getting any hints about their story/past without it.
I don't need to read the Onyx squad's diaries to know that something happened between them and that there was a huge conflict, and who is on whose side.
But ZEL don't give that insight, so I agree with you that they're poorly handled there.
I think that if you take away the diary, then Zero and Lumi are still good characters because you can easily get an idea of what kind of people they are. I mean, with Zero especially, the largest part of getting to know him relies on his dialogue, since the diary doesn't really tell us much.
PULSE Muk was actually a memorable conversation to me, because I really enjoyed that brief moment of Zero and Eve not arguing against each other for once. I wouldn't call it teamwork, but having them not bickering for once was interesting.
And personally, I'll disagree entirely on PULSE Abra, because to be honest, I only kept Lin at the back of my mind until she actually appeared. Was I intrigued to learn about her existence? Sure. But I was very much distracted by the actual situation at hand, and it gave me what I think is ZEL's best appearance so far.