To be fair, he's working with Elidibus. Anyone who's taken a few minutes to hear that guy out tends to peruse that pamphlet.
But I'd agree that Varis is malleable. In Patch 2.4, they reveal that he was so set upon Eorzean annexation that he spoke out against and clandestinely worked to undermine the Meteor project. When discussing the Ivy's masters, the Alliance concludes that one must be Varis because it's a higher authority than Gaius van Baelsar but it can't be Solus because he cared more about exterminating primals than conquering Eorzea to the point that he would see it reduced to ash.
Fast forward two patches.
So yeah, Varis has shown that he's adjustable.Varis zos Galvus
You know as well as we what will ensue should these insatiable creatures be allowed to roam free─that their very existence threatens the life of this star. We but disagree on the solution to the problem. <...> The fate of Eorzea and its inhabitants is of little concern next to the fate of the world. 'Tis my solemn charge as emperor to bring the eikons to heel. If this requires the extermination of certain elements, then so be it.
I'm no Garlean sympathizer by any means, but imho it's to be expected that the people we've talked to about the empire (territories far from the capital, populations in open rebellion, hamlets with no strategic value) have many stories of neglect and slaughter. We've at the same time been told that cities which transition well to actively contributing to the "save the planet" bandwagon barely change once annexed. (Though who knows what's true.)
As for Solus I think he was just old. When you're pushing 90 and have been stalled on the last leg of your goal for over a decade, some things become acceptable that might not have otherwise been so. He was sick and dying and Nael offered him a chance to end the primal threat on Aldenard. He didn't care about (what were to him) a bunch of beast tribe savages and petulant, false-god-worshipping city-states who actively fought against what was best for them. Gaius van Baelsar was disgusted that Nael had so deftly deceived him into hand-waving the risks.
I agree with all of that, just a few degrees askew. I think Varis is likely to screw up, though I'm not sure if it's more probable that it will occur before, during, or independent of Eorzean intervention and/or Garlean power struggles. I think that we're going to Ilsabard in 5.0 and that it's not necessary that we arrive on bad terms if everything unfolds in the background. I think the core of the Empire is probably happily Garlean, but that Varis has not been behaving in concert with established Garlean values; I think they might be open to his removal if he crossed certain lines (some might be content with that based on lines he's already crossed if they found out about it). And I think at the end of 5.0, the Garlean Empire should be intact, strong, and with better leadership to best prevent the spread of further war and primals; and aid in the final battles.
From a gameplay standpoint, they also have to make sense of open-world enemies. If we made peace with the Garlean Empire today, being aggro'd by soldiers in Yanxia and La Noscea makes sense. They're remnants of rogue, fallen, abandoned, disgraced legions. But we can't go to Ilsabard on bad terms, make the context that we're there to fight imperials, and then make peace with them and leave a bunch of aggro in the field, lol. They're not spoiled for choice in how to resolve this conflict (or good excuses to go to the Near East and do things, really).
That doesn't mean war must be protracted, though. Carving a path to the heart of a problem and destroying it is something we've been able to do before - including doing it to both sides of a war that'd been actively raging for 1,000 years. We don't need to shatter Ilsabard to have a few battles and throw Varis off the throne if need be. We killed the leader of Ishgard while wandering the city-state and counties in relative peace. Deft use of instanced skirmishes counts for a lot.
Why would I focus on this for the time being? Thematic consistency, I suppose. Since 1.00, there's been a supernatural war (Ascians, primals) and a traditional war (Garlemald). The forces behind the supernatural war often exacerbate traditional wars to achieve their goals, and this was to be no exception. However, after all that anticipation and framing, should the story really just allow the entire [War with the Garlean Empire] to evaporate after we gnawed away at two weakened fringe cities and killed the son of an emperor with fewer boundaries and more atrocities than the last? It's not terribly cathartic, lol. On the other hand, we don't have a great history in taking out Elidibus's pawns with our own hands, either.
Still, a guy makes artificial Echo users by aether-sucking (at least) hundreds of people to death while simultaneously turning his own people into Tyrants-a-la-Biohazard and working to bind primals, I'd like to personally kick him with my own boot and end the war on that note. I'd also accept Gaius van Baelsar crashing an airship into him.