I'm not sure to what extent I want to see Zero as truly being dedicated to Meteor's ideals (no matter whether we're talking Lin's or Solaris' here). We simply don't know enough about him to know what motivated him to join in the first place.
I think as far as his position in Meteor goes, he's really more of a case of a grunt being caught in the crossfire (his rank prior to ZEL has never been mentioned, but I don't think he was an Admin all along), so not on the same level as, say, Sirius, in terms of devotion. (Personally, I like to imagine that he's more of an Aster case, someone who joined for the personal gain, someone who comes from a background of having nothing.) (I'll admit I really like Zero. I like the outspoken, stubborn attitude he has, and I'd be lying if it didn't amuse me every time when he gets upset over things not going the way he'd like them to. So maybe I'm biased because I really want him to side with Eve and Lumi.)
I think that as far as ZEL's dynamic goes right now, the ultimate decision-maker is Eve. Lumi looks up to her for that, and as stubborn as Zero can be, it's been shown multiple times that he fails at challenging her authority.
Right now it would seem to me that they're in a situation where Eve would respect Lumi's opinions if she gave any, but at the same time, Lumi trusts Eve to know what's best for them.
It is obvious that Lumi is very uncomfortable with what ZEL do, and I'm sure Eve knows that. But what Lumi would need to do to cause any change in their behavior would be to firmly put her foot down for once.
I'd love to see her to that, but I'm not sure how likely that is.
I am not so sure that we're going to see ZEL in the next episode. From what it seems, it will take place in Reborn City, and they have no business being there. Meteor isn't currently maintaining any PULSEs there. I'm guessing ZEL are more busy with the ones creating the barricade around Labradorra.
While the teaser for E16 said that "the heart of reborn will fall", implying a possible attack, ZEL aren't what I'd call an attack force.
I don't think Meteor needs a PULSE for it, so unless they do, there's no business for ZEL to be there.