From what I've been able to piece together, Icarus was a NEONet researcher, and something forced him out of the corp. He's intimated that he was involved in Project Imago, but is suitably unwilling to divulge details on his past.
The 'Them' the message was referring to was pretty clearly not the Consensus, but Pulsar and his allied AIs.
IMO this sentences:
Colleen, I’m really pleased the Consensus adopted
your policy proposal. I tell you, better to have them on our
side than against us.
Speaks about one and single subject... Consensus is "They" -
It is better to have Consensus on our side, so it is good that they adopted/accepted your policy proposal.
Pulsar and his allied AIs being consensus are exactly what I had on my mind when I first posted this 
The 'them' is most certainly the AIs. However, that does not mean that the Consensus is also 'them'. The context you're missing is that the memo is clearly a continuation of conversations these two have had either online or in person. There was a proposal sent to the Consensus regarding the AIs that had contacted Horizon, and the Consensus approved it. The person who sent the memo was glad of that, because, as he's likely said before, it is better to have a bunch of AIs on their side than against them.
While all Horizon employees feed data to the consensus, and sometimes participate in consensus initiated votes on some decision or other, i have the impression that it seems to be a distinct entity or group. There is also the opening fiction in Conspiracy Theory to consider, where Fastjack laments about overlooking the human element. Now if that is a reference to the consensus or the analyst that brings jackpoint to their attention, i am not 100% sure.
I'd say that the human element in this case is NOT the Consensus. The Consensus is described as a massive social network dynamic model, updated constantly by the data input by Horizon citizens. There may be (probably are) multiple AIs and human system techs who manage the data, and certainly the hardware that supports it, but the Consensus is the actual model, which predicts the responses of Horizon employees to any given situation.
I know I mentioned this someplace else, but Iris Firmware did the Athabascan Council's matrix. It was all wireless back in the 50's using satlinks and microwave transmitters. Iris Firmware was a subsidiary of Renraku so it is quite likely that Deus knew about it. S-K had some sort of safety switch that cut them off from the matrix during the Crash 2.0, but the Athabascan network would have been relatively fragile and thus the first hits would have isolated them at the hardwire interface. I.e. The Athabascan matrix would have been just as safe as S-K's.
Deus could have migrated his primary database up into the Athabascan grid and then run essentially clones on the main matrix. The nano technology being developed in the SCIRE could have then offered modular platforms to host Deus subroutines. Those modular hosts would however need a larger wireless network to interface with the main body, thus the wireless matrix revolution and proliferation of commlinks.
The greatest trick [Deus] ever pulled was convincing the world he didn't exist.
First, that wasn't 'some kind of safety switch'. That was Lofwyr putting a back door in every utility Saeder-Krupp ever touched, so that he could, with one claw, shut down any government that annoyed him too much. The governments of Europe simply didn't know it existed until Lofwyr showed them the knife at their throat.
As far as the Athabascan Council's matrix, Iris Firmware would be acting under the aegis of Renraku America, which means that Deus certainly would know about the state of the matrix there.
You know...I understand that Deus may be some kind of favourite oponent during the time, well it coexisted with multiple AIs later, fought with them, was defeated. Maybe one day it will rise again, to be defeated again. But its supremacy is far gone, with sprites and matrix realms etc...and Deus as an Artifical Inteligence is not resonance being, so it has no acess to realms itself...only it it is some kind of primordial supersprite...
Actually, Deus was fighting two other super-AIs, plus any mortals who cared to play. And all together, it was only unleashing Jormungand that made him disappear. And the Resonance Realms were there before, back when there were Otaku. It didn't make Deus any less of a power in the Matrix, or a threat to the physical world.
