Why attack the Ark then and want a Halo ring? That’s got nothing to do with Empire. That’s consistent with Atriox dealings with the Endless and him wanting to take Zeta Halo. The whole of Halo Wars 2 is based on him wanting to use a Halo ring and this being terrible for humanity. How is that not exactly what he’s doing in Infinite? There’s no escalation from the Banished between Halo Wars 2 and Infinite; it’s the same end goal. For a change to have happened as presented in Infinite this has to have been just after Halo 5. Otherwise, firing a ring they got in the Ark would have destroyed Doisac and killed all the Brutes.
Of course it does; power. The Ark housed a lot of Forerunner weapons, and Atriox knew that would be a great haul. As for the ring, they were just as surprised to see it as the UNSC was to discover it. But again, controlling the ring ensured controlling the Ark, was a way back to the Milky Way with the loss of his ships, and would thus be a way to control a large part of the Galaxy.
Before Doisac, I significantly doubt that the Banished would have assaulted the Infinity like that unprovoked. As well, the goal in Infinite became that of burning it all down. Making the UNSC suffer as the Jiralhanae had with the loss of Doisac. The Harbinger and the Endless are a means to that goal. The escalation is that the Banished never directly went to war on that scale, and on December 12, they did.
It would have killed all the Jiralhanae in the Galaxy, yes. But to that I doubt the intent was ever to fire the ring. Just to control it, and have that threat. It’s like the Cold War; no one really wanted to launch the nukes, they were just there to give weight to the threat.
They massacre every human civilian on the Ark. It’s an act which only makes sense if they are already on their anti human crusade. It’s not out of character to attack the UNSC with incredible violence. They also attack the civilian population on Reach. If the switch flipped when Doisac is destroyed then it has to have been before these events.
Some of the ambient dialogue from the UNSC characters imply, I think it’s Isabel, that he’s after a Halo ring. He also becomes determined to access to it at the end of the game, to the point of recklessly throwing an entire army away in suicide charges. That stacks with his obsession in obtaining Zeta Halo in Infinite. He had a slightly more benign reason in HW2 but two weeks ago it changed and he now wants it for wiping out humanity. That’s too messy. It makes a lot more sense if Halo 5 ends, she destroys Doisac and that’s the instigating event of Halo Wars 2.
Why would he be bluffing? There would be no threat if the character was bluffing initially but has changed his mind now.
Which makes it a huge plot hole. In Shadows of Reach there’s no reference to Doisac being destroyed and that’s basically right before Infinite begins. The more likely answer is they didn’t tell the author they planned on doing this to maintain the shock value when it happens in game; even though it should be common knowledge. Even in Infinite itself there’s very little reference in dialogue or cutscenes to this apart from at the House of Reckoning.
I mean when Esharum said he lost his honeworld and got stronger but questioned whether Chief could say the same I had questions… 
I mean, there was really only the one research outpost. And it was less that they massacred the site, but Decimus did. I’m not saying Atriox wouldn’t have done the same, and certainly it wasn’t done because they were human, but more that they were in the way and easy to dispose of so that a Banished foothold could be had. They are still Jiralhanae, after all.
As well on Reach, their primary goal was to secure the portal site. I’m not saying they were entirely peaceful - again, their still Jiralhanae - and they certainly eliminated Militia opposition when presented with it, but their goal there wasn’t eradication of humans.
It’s mentioned in the mission “Ascension” that the Banished are looking for the Cartographer, which is “the map of everything in The Ark; that means Atriox could use it to locate key systems and weapons on this installation”. In “The Cartographer”, it’s learned that Atriox was using it to access and take over the Ark’s portal systems. In “Lights Out”, Decimus boasts that the Banished control the Ark, and that none will stand before their might. This lays out their purpose there as further gathering power and weapons.
It’s in the cutscene for “Hold the Line” that Anders suggests completing the near-finished Ring, disabling it’s weapon systems, and using it to make it back to the Milky Way. In the closing cutscene for “Under the Dark”, Atriox is just as surprised as Let 'Volir, and commands him to take it from the UNSC. In the mission “The Halo”, Cutter voices his concerns that “If Atriox gets control of this Halo, he’ll be holding a gun to the head of all life across hundreds of light years.” Which isn’t to say that such is his goal, just the UNSC’s fears. Atriox does mention in that level that if the Spirit of Fire leaves now, he won’t hunt them down, which does indicate some of his plan, at least that it’s not “get the ring, end all life”.
And that’s really all that’s mentioned of the Ring in the main campaign.
His obsession with obtaining Zeta Halo was to provide a fortress world for the Banished; they constantly refer to it as their new home. I don’t think his goal is so much wiping out humanity through the Endless, just more tearing down all the lies that have been built up.
Having Doisac be the catalyst for Halo Wars 2 doesn’t make much sense at all, though. If that was the case, Atriox wouldn’t have offered mercy to the Spirit of Fire, nor would he have allowed any of them to live or run. It would have been the Infinity on a larger scale.
Likely because it happens after Shadows of Reach ends. That or, as you note, it’s a massive plot point to be saved for Infinite; you don’t give away that big of a twist in media leading up to the main course.
But even then, it wouldn’t be common knowledge to any but the Banished. It’s not like thy would tell anyone what happened, and the UNSC’s communication was in shambles, especially after HIGHCOM was destroyed.
I don’t remember Escharum mentioning loosing his homeworld before it was revealed, though.