On Solus...
Well, now this is amusing!
Long have I suspected the Ascians had a hand in Garlemald's rise to power, but I didn't suspect it went beyond a simple nudge - giving them the technology to build warmachina, chiefly. Instead we now know that it went from a struggling backwater to a global superpower because of direct Ascian manipulation, and that he fed them lies the whole time to get them doing what he wanted! Eikons causing the Burn... the racist garbage... their "civilizing mission"... doing so to put eikons under their heel and keep the world safe... nothing but a bunch of meaningless lies!!!
Ah ha ha ha ha ha ha ha...! Well played. Well played, Solus!
Anyway, let's set out what seems to have actually happened.
"Solus" accomplishes what he does by introducing the Garleans to warmachina, then continues onto Othard and the Burn, showing the Garleans the effects of aether starvation. He lies that this is the result of eikons (none of which were known to have been summoned in the Sixth Astral Era before Silvertear!), retroactively justifying the Garlean conquest of Ilsabard and justifying further conquest under the idea that the world must be brought to heel. From there on, he doesn't really need to do anything - Garlemald bought the lie, and with that in place will continue to grind its war machine against the world under the belief (heh) it's what's right.
Meanwhile, after the disaster at Silvertear and Midgardsormr's death, other Ascians (and perhaps "Solus" himself) teach the Beast Tribes summoning rites, which they use in response to Garlemald's aggression, "justifying" that aggression, leading to an endless spiral of violence. At this point the (Legacy) Warrior of Light enters the picture, forestalling the Garlean advance and quelling the Beast Tribes. When Nael offers to use Meteor, "Solus" accepts and doesn't look back because it's exactly what he wants. Though it wasn't a full success thanks to the future Scions and Louisoix's sacrifice, it was enough to accomplish his mission - and with that out of the way, his aging mortal body, and everything in place for Garlemald to continue stirring up trouble, "Solus" leaves the stage.
Only... that's not what happens.
Lahabrea's arrogance fails them twice, getting himself and Igeyorhm killed. Varis, after winning the war of succession, focuses more on stabilizing Garlemald and building his power base. The war machine stalls. The Warrior of Light and Scions' efforts begin to spread peace, and even some Garleans want in it. All of his efforts are being threatened.
So... along with Elidibus possessing Zenos' corpse so they can kill him and assume direct control if Varis proves uncooperative, "Solus" comes back and tells Varis the truth about the Empire and its mission. This development puts Varis in a vicegrip - he can either comply with the Ascians' wishes willingly, or defy them, get killed, and have the Ascians assume direct control (again).
Varis, on the other hand, earnestly believes the lies he was fed growing up and wants to make that fiction into reality. There was no need to know about "Solus" or the Empire's true nature, so long as they fulfilled their purpose - so why would he know the truth?
On the one hand... it's clear Varis is now in over his head, two Ascians holding a noose around his neck and making him dance to their tune. On the other, it's... fairly certain that without said noose he'd still be gunning for universal domination, just not at a pace the Ascians find acceptable. (More accurately he wouldn't be trying to create conflicts all the time, but he still wouldn't have trouble with using violence against those opposed to his ambitions.) I must admit, I'm quite curious as to how this is going to play out.
How... very... amusing...
As for how much the Shadowhunter (totally not Gaius) knows? None can say. Were I a betting man (and I assure you I am not), I would wager he knows there's Ascian corruption in Garlemald but not quite the depth it goes to. Makes me question whether Lahabrea approached Gaius of his own accord, or "Solus" hooked him up before leaving the stage...
Let's just see... heheheh...