Are there two versions of the Eos, then? One that the Atreus / Imperiuse use for SSMs and another for tactical engagements?
Yes, missile launchers on Tev destroyers are specifically said to be able to launch different kind of missiles. ("One of a number of munitions that can be launched from the Raynor's multiple missile batteries, the Eos is a long-range weapon..." quote : Eos tech room description).
Was the capture of the Agincourt all part of the plan (Steele's plan to moustrap the UEF strike on the Carthage) from the beginning? Or did he create the plan afterwards?
Opinions diverge on that point, but my personal opinion is that the Agincourt's capture was NOT part of Steele's plan. However, he DID have contingencies in place for this or a similar kind of event, in this case, asking for Vasudan support, which leads to free supplies that can't be actively targeted by the UEF for diplomatic reasons, so don't need heavy escort like his own supply chains. Which means Steele is actually in a better position strategically AFTER the Agincourt capture than he was BEFORE.
tl;dr : Steele didn't plan it, but he masterfully used it to strengthen his position.
Was the agent actively working for Steele and misleading the UEF? Or was Steele feeding misinformation to the agent?
The mole was, or at least is strongly hinted to be, former Rear Admiral Thea Carey, former commanding officer of the GTD Temeraire during AoA (see
Conversations from War in Heaven, the "Bergher-Tanikawa Suites" part). She gets arrested, probably after they found out she was the one leaking the intel on the Agincourt's jump schedules ("You gave them the ****ing jump schedules, you traitorous piece of ****. There were ten thousand people in that -") (which is one of the main reasons for my opinion above), and Steele then faked her identity as a mole to feed the UEF misinformation about the Carthage.
The only reason I ask this is because the Agincourt plan was predicated on the absence of the Imperiuse... and if you think about it, when the toutatis jumps in to wreck the hood, the Imperiouse and a couple of well places strike corvettes could've jumped in and wrecked the **** out of the Toutatis. Maybe not outright killed it.. but a flank jump with all that forward firepower would've done some major damage.
Well, Steele probably thought it was better to keep the Imperieuse for a better opportunity strike where it hurts. Sure, loosing the Big T would have been a huge victory for the GTVA, but arguably striking the master blow to the Wargods was a much, much stronger blow to UEF morale. Besides, heavy jamming in the Aristeia combat zone from both sides probably means Steele didn't had an accurate or reliable intel to send the Imperieuse in. Did he even know the Toutatis was there ? How could he know the UEF didn't have a couple of Narayanas or something ready to wreak the first destroyer that would show its blue nose into the fray ?
Not to mention that Steele might already have decided by that point that it was actually better on the strategic point to loose the Agincourt, and gain Zod support, than unveiling his master card.
On that last point: I don't think the UEF actually touched the Imperieuse, it is simply said that it retreated to Delta Serptentis to resupply.
The Imperieuse led Darkest Hour's blitz ("Galactic Terran forces of the GTD Imperieuse battlegroup have overwhelmed our defences at several key positions." quote : DH briefing, "The GTD Imperieuse's squadrons blitzed the entire Sol Sector in a single, massive coordinated strike. " quote : DH debriefing).
"The Imperieuse has withdrawn to Delta Serpentis to restock its squadrons." quote : DH debrief.
"The GTD Imperieuse has withdrawn to Delta Serpentis to replenish its squadrons after spearheading the Blitz against Earth." quote : The Plunder brief.
"The Hood is still undergoing repairs from the thrashing we gave it, and our source says the Imperieuse is still in Delta Serpentis resupplying from the Blitz." quote : DE brief.
So yes, nothing says anywhere the Imp took a significant beating. Since the Imp didn't have access to a drydock during her time in hiding, if she was wrecked, she wouldn't seem intact at DE. She could have taken a few shots they managed to repair in-field during all that time though.