On the Burn...
Irrespective of its actual history, Eikons/primals are known to exert a draining effect due to the amount of aether they consume. No one now denies the need to check their summoning and it is far, far easier than ever imagined, as Ysayle, Thordan and Tsukuyomi all proved. It's unclear when Solus became possessed by an Ascian, but it is now plausible that there may have been an Ascian hand leading the Garleans into the Lake of Tears, who did not know of Midgardsormr's presence. The Ascians probably calculated that he'd react in the way he did, resulting in summoning growing into a larger threat, further bolstering the Garleans' resolution to quell it... an Eikon like Thordan could still, nonetheless, turn the world into something like the Burn.
On Varis...
It is pretty clear to me that he had taken to heart the Empire's stated objective and sought to act on it - he truly believes in it and has yet to come to terms with what "Solus" revealed. Whatever the nature of Solus, Varis sought to act on the ostensible ideals upon which he built the Empire. This is consistent throughout the story where he appears; "Solus" even remarks that he is an easy one to read, no doubt due to his direct nature. The real question is when did the Ascians begin ramping up their manipulation, and on what bases? "Solus's" tone is highly suggestive of Varis only recently having been made aware of the truth about the Empire. Like Remedi said, this new Ascian was not relevant until recently, i.e. after the events in Azys Lla - incidentally, this is also when Regula encountered us in the Warring Triad side-quests and said to the WoL that the Emperor may have need of his assistance in the future. Quite when did Varis know he was an Ascian? No clue. Could've been well before, and perhaps that's why he spat on his uncle or grandfather's coffin, thinking that'd be the end of it, without being as yet fully apprised of how much of Solus's life was spent as an Ascian. For now, we don't know and as usual, there are a great many variables.
My view on what has happened is that Elidibus was playing "good cop", even allowing Varis to pursue his Ascian brethren, and making promises to help the Empire fulfill the objective Varis had taken as his own (primal eradication), but that doubts may have begun to arise when Elidibus took on Zenos's guise. Hence, Elidibus rolled out this new guy, who eventually - and presumably, recently - fed Varis some cold truths (assuming they are, since these are Ascians), and we have the status quo, hence his bitter resentment. There's I'm curious, though, what "Solus" meant when he said Varis could accomplish a great deal to further the goal of dispatching threats to the balance, not just through the Empire but his own hand. An intriguing remark. Was he trying to tempt him to take up the Resonance? Will this new guy turn the tables on Elidibus at the first opportunity and become a villain, with Elidibus being more of an antagonist? Would be an interesting twist.
I think if someone is hellbent on seeing Varis as a villain or irredeemable, they'll do just that, just as many did with Gaius, Regula and many other characters. I disagree with this interpretation of where SE is taking the story and I am hoping SE will be putting him to more ambitious uses than that, i.e. a reluctant ally and "in" into Garlemald, maybe when he sees Alphinaud accompanied by Gaius, a man who was in a situation much like he is in now. We can but speculate given how little we know.
On the Garleans...
You have to ask, why would the Ascians specifically choose them, when they are mostly unable to wield magic, a tool Ascians utilise to great effect, as did the Allagan?
I suspect it is because they are the same people as those who left Goug, and therefore their technological prowess was known to the Ascians, making them uniquely well suited to utilising and reverse-engineering Allagan technology, and having a motive to do so, due to their aetheric deficiency. There is no inference that the Empire was solely possible due to the Ascians; only that they had used it as their project to sow chaos - if anything, they may have sped that the process up. What I think they really did was to provide the rationalisation for subjugating territories beyond Ilsabard. Garlemald already had gained the upper hand over its rivals prior to Solus turning it into an empire, and probably would've taken over Ilsabard in time - what the Ascians seem to have done is galvanised the process, sped it up and cast its gaze outwards. No timeframe has been provided, so this is ambiguous until SE clarify further.
I also believe the Ascians considered their widespread inability to wield magic a tractable problem - through the Resonance. But with the cat now out the bag, Garlemald may eventually slip from their control once the truth gets out. It is also why I do not think the Ascians have any appetite to kill Varis and risk a war of succession. This may be the role the real Zenos plays - to expose their deceit. However! The counter-maneuver proposed by Thancred is easily predictable, so I have little doubt that Elidibus will factor this in, if he's not already aware.
On the Ascians...
My pet theory now is that, as beings whose deity is not native to/embedded in this Shard, there is the possibility that the power they enjoy as servants of Zodiark is limited by the potential of their host to channel aether. That said, it would appear to me that for the Overlords, they have a great deal of control over their host's form whilst they're tethered to it. So much so that they're able to bring it back into being if it is injured, merge it into a form like the Ascian Prime (the motive being, to surpass the limitations of the host), and probably do much more than just body hop. Elidibus's comments on Zenos's "adamant" form are unlikely to relate to Zenos's strength alone - we know from the Nabriales/Moenbryda interaction that they can augment their host's strength to incredible heights. I suspect it has more to do with the raw ability to channel aether and allow them to more fully manifest their Echo and other powers. Zenos was, after all, enhanced through the Resonance, most likely an extension of Zodiark's echo. Again, the Pureblood aetheric deficiency might make possessing them less susceptible to interventions by Hydaelyn. This is why I also don't buy that "Solus" was doing anything but performing a ruse on Varis. Pretending to die by letting the host perish, knowing full well that it would not suffice to override his control or "eject" him, so he could just conjure back the body seconds later and spook the audience. The fact that the fight with Nabriales took place in the Chrysalis might be a slight wrinkle here, but it could be because he was already tethered to a host and had to act fast - additionally, I think the newer lore is overwhelmingly pointing in this direction, which would be in common with how fantasy settings tend to deal with extraplanars. We know little of the true nature of the Ascians.
On Hingashi...
Theodric's point about Val is also a good one. I had forgotten about it and it is one reason why they may have very little taste for joining anyone remotely associated with the Eorzean alliance, for now at least. I doubt it enamoured them too much towards Doma, a nation they perhaps might consider a prospective competitor for influence in the region, once it rebuilds.
Hopes for the future...
I am really looking forward to this new Ascian playing a big role and I hope that he is very prominent and theatrical in his execution, like Ardyn. The scene between him and Varis also reminded me of this scene from Batman Ninja!
https://www.youtube.com/watch?v=OSInDJ6or2o
I'm also very hyped now to see where the Scion storyline is heading, especially if this is Zodiark with whom we're communing.