So if we can't start top-down, why not start in the middle?
There are a lot of Forgeworlds out there, a lot of Fabricators and Archmagi with influence and independant ressources and knowledge.
If Horus could draw a significant faction of them on his side as a little side-project besides his other Heresy-preps, why couldn't we?
Offer the leaders of Graia, Cypra Mundi and other important Forgeworlds knowledge and archeotech or even a bit of Mona's time in exchange for supporting our Campuses and other policies and see how many of them let their love of the good loot overwhelm their conservatism.
Of course this takes longer and is less efficient than getting the help of Mars itself, but it's still helpful in the mid-term, and in the long-term it might even get us enough AdMech backing not to be forced to negotiate with this particular FabGen anymore.
I think this might be a good plan. If you want to work with the Admech, working with the Fabgen and strengthening her with repeated concessions that she will latter use to bury us and her rivals in admech is not a good idea.
Instead work with the people from the opposite factions from her and especially other forge worlds. Help increase their independence and power. We may not be able to disrupt the treaty easily but it would be more viable to weaken the orthodoxy and dominance of Mars as the foremost leaders of Admech. Fragment the entire thing preferably and annihilate centralized control.
It's a goal that the other forge worlds may prove more sympathetic toward compared to breaking the treaty, even if it is just out of self interest and desire for more autonomy.
This could theoretically work... If not the fact that Antegymax has the rest of the Mechanicus in a pneumatically powered adamantium vice grip, and neither Pandora nor Mona are Mechanicus Reformer Sensei. So the gains would be so minimal to not be worth it in the first place, or outright non-existent. At least with the effort we can
afford to allocate to the Mechanicus-problem.
Which is why we are here, making a deal with a political opponent who knows that Pandora will not just delete her existence when Pandora's morals and vision for the future are taken into account. Also time, because the Psychic Awakening is coming, and we cannot afford delays in dealing with it. Which trying to solve the Mechanicus the hard way right now would inevitably involve.
Anyway, I would like to remind that there is an upcoming social with Lyra and her master, who are quite the way into the radical-spectrum in the way that that is more palatable to us and Pandora. So if we want technological expertise that aligns more with our vision, there is our chance. And yet at the same time they still can't replace the support of the Adeptus Mechanicus when it comes to dealing with the Psychic Awakening in time.
Nor can they give us the same kind of force-multiplier that a Gloriana-Class would be in the short-term. Which I think we kinda need if we want to pull of the Sanguinius + Ynnead -combo in the next turn without horrific attrition on our side, or potentially even suffer an outright failure if things escalate too rapidly.
Basically, Lyra and her master might give us the seeds for some form of long-term solution, especially if we put Mona on task at some point to get them started. But that is in the long-term, and we still need the short-term solutions to get to enjoy the fruits of long-term efforts. Even if doing so will cause problems in the future.
(EDIT: various typos corrected and wording changed)