Rising Poem
I think the Rising Poem is talking about two different individuals, both of who have the Echo. However, they use it in different ways. The first Echo-user is using the Echo to shine light around the realm while they travel over it. The second Echo-user is using it to sow darkness and confusion by transcending the differences between people (possession). It's very possible that the Poems is talking not about different individuals with the Echo, but different groups of people with the Echo. Both the WoL and the Ascians have the Echo, but they use it in very different ways and both the WoL and the Ascians have influenced both the situation in Ishgard and the situation in Ala Mhigo. Both of them are definetly going to influence the situation in Garlemald.
Balance (or unBalance)
Everything we've seen in the game (and from lore interviews) up until now indicates that the world has not been in balance since the Astral/Umbral Eras started. There's a lore interview somewhere (I can't find it, anyone knows which one it was?) where Koji revealed that when Hydaelyn sealed up Zodiark in the moon and shattered the shards, she did it out of desperation because she was weaker then Zodiark at the time. And that ever since then, the Ascians have been trying to rejoin the Shards to Hydaelyn which, once finished, will wake up Zodiark again. There's literally only one character in the entire game that does not act like this is what is really going on (Elidibus), but given his particular brand of "leaving out that one piece of information that turns the meaning of everything I told you on it's head" I'm not trusting him. Especially when the being whose influence he is trying to lessen is operating in a "all life is sacred to me, even if letting that shard be flooded with light would deny my enemy strength" mode.
I think it's safe to say that no one knows what a balance between Light and Darkness actually looks like. Maybe not even Elidibus. For all we know, he's as manipulated by Zodiark as Elidibus' pawns are manipulated by him. Further complicating this is that for a "true" balance of Light and Dark to exist, it might be that the word(s) as we know it might need to end. I think it's safe to say at this point that Hydaelyn stopped settling for a "true balance" in favor of "enough balance" so long as her children have the opportunity to live out their lives without Zodiark or Ascian involvement. However, the Ascians are all about meddling in people's lives and from what we can tell, Zodiark never wanted balance in the first place. I don't think they are going to stop until Hydaelyn doesn't exist anymore. Elidibus might care about balance, but he's willing to sacrifice everyone to bring it about.
Garlean Stuff
The Garleans have a major problem on their hands, that they should really be paying more attention to if they want to keep their cultural and technological improving. Their best engineers, their best playwright, and arguably one of their best generals have all decided that it's better for them to not be working with their state under the current regime. I can't imagine that they're the only Garleans who think that, it's just that they're the Garleans who had the means or opportunities to flee Garlemald and get away with it. I fully expect us to meet more Garleans who want to leave Garlemald in the coming patches. For me, Garlemald has gotten to the point where I'm less concerned with people betraying the country, then I am with the country betraying it's citizen's ideals for it. Once a country has gotten to that point, leaving it can be as much a deceleration of loyalty to what the country used to be as it can be a betrayal of what the country is turning into.
Cid and Nero summaries/ramblings (under spoiler)
I feel like calling Cid (and anyone else fleeing Garlemald) a traitor would be more okay if he was the only one. But he's not. Canonically, it's not just him who leaves Garlemald, it's him and a lot of the other engineers working in Garlemald with him who leave all at once. Cid leaving Garlemald is more like when Jenomis cen Lexentale left Garlemald. Or rather, I wouldn't at all be surprised if Jenomis decided it was time for him and a lot of the people in his theater company to do what Cid had done years earlier. In fact, both of them leave for similar reasons. Cid leaves because he can't stand to work on only the military projects the government wants him working on. Jenomis leaves because the government is dictating what he can't wright about. Both of them are creative geniuses whose government is interfering in their creativity. And generally speaking, when the "best of the best" of a country's creative talent decide to leave, that's not a good sign for the countries creative output. And modern Garlemald was built on the creativity of it's engineers...
Story-wise, I feel that Cid is a lot like Minfilia in that both of them got a lot of character development in 1.0 that is vital to who they are as a character, but then none of that character development is in 2.0 even though the game kinda assumes you know that it happened. Given what 1.0 reveals about Cid, I have a very hard time viewing him as a "weak" character, quite the opposite in fact! Cid is pretty much the son of the scientist who was the head of a military project that made WMD. The scientist is so obsessed with his project that he all but lets his best friend, one of the legati in the military, adopt his kid. Meanwhile, Cid graduates from the best engineering college in his nation, and is expected to design technology for the military since that's what all the good engineers in his country do. However, something goes wrong on the project his dad is working on and blows up the entire city the project is based in. That drives home to Cid what he is really working on and when he finds out he's been given his dad's job (the job that ultimately killed him), he decides to leave since his government/military don't take "no" for an answer.
However, he doesn't just get himself out. Many of the people who he works with don't like what they're doing either, so he gets all of them out. Instead of fighting directly against his native country however, Cid decides to go someplace that currently isn't in direct conflict with it. He sets up shop in the middle of nowhere and starts producing technology that has nothing to do with war and everything to do with making people's normal ordinary lives easier, which is something he never got to do before. In addition, the company he founds still has ties back to organizations in his native country that help people get out of it. However, his exposure to military technology (and the mindset behind people who want technology to be made for the military) has left him hypersensitive to what people can do with technology. He really doesn't want anything like WMD to be made, so he has a tenancy to be paranoid about what might or could be used to make it. And there's a lot of tech that could be used like that all over Eorzea...
Now, if only Cid's introduction in 2.0 was more specific about what was going on then it is... it glosses over so much it's almost a disservice to the character...
As for Nero... heh... I almost doubt he was ever loyal to Garlemald to begin with. He's the complete opposite of Cid in terms of upbringing. The Lore Book has him growing up in the backwoods of Garlemald, which he wanted out of. He puts in a ton of work and manages to get into the Magiteck Academy. There he shows a talent for reverse engineering ancient technology into modern technology. However, he's completely upstaged by the son of the best scientist in the nation, who for all practical purposes might as well be Garlean nobility. Nero gets annoyed by this and starts up a rivalry with Cid. Cid however, is completely oblivious to it. When Nero graduates, he get's offered a research position at the Academy, but there's a problem. Nero is obsessed with Allagan tech. The organization that gets first dibs on Allagan tech isn't the Academy. It's the military. So Nero turns down the position at the Academy and goes into the military instead. He doesn't just settle for being a good engineer, though. He manages to become the second-in-command of a legion. Coincidentally, the legion that is run by the legatus that all but adopted Cid. And if said legatus decides he isn't going to be showing up for that civil war going on back home? Well, at least chose a land filled with Allagan tech to take over instead. Nero's up to his eyeballs in tech to research, which is why he wanted out of Garlemald in the first place. He's probably wouldn't be going back to Garlemald even if he didn't have a death warrant waiting for him back home. That he as the opportunity to show up Cid again is icing on the cake.
Why yes.. I do think Cid and Nero are foils for each other...