Short version: Delegation.
Long version: Despite Avernus being within and part of the Valinor sector, it is not the job of the Lord Sector to do the things that Frederick has been doing, i.e. the job of a Planetary Governor. In similar fashion, Hye Richards does not personally oversee the Guard of every region but instead assigns subordinates to oversee them for her.
This is where Fabricator-General Scott fails.
Part A
Her idea, on the surface, is not a bad one. She can only do so much at once and having the Administratum help out in expanding the Forge Hives will take some of that load off her. There are Forges in the Asgard sub-sector beyond those found in the Avernus system and they are themselves run by Fabricator-Generals. Scott's reform will free up their time just as it's freed up hers. Efficient, right? Not quite.
The fact that non-Mechanicus personnel can oversee the expansion of a Forge Hive is proof that a Fabricator-General is not a necessary component in doing such a thing. That job can be given to someone else. Why, then, is it given to the Administratum and not simply to some other member of the Mechanicus? (Perhaps a Forge Master or maybe make a new rank to fit the job?) It would free up the Fabricator-Generals' time, it would allow the Administratum to perform other duties, and it wouldn't cost an additional +10% to expand the Forge Hive.
For us, this would theoretically go one of two ways.
1. A new spoiler opens up under Adeptus Mechanicus (in the regular turn sheets) dedicated exclusively to this kind of expansion work and with it a new Adeptus Mechanicus action to be spent on it. You've made your position clear on giving additional actions so this isn't viable. (And just to be clear, I absolutely do not hold this against you in any way. You are doing nothing wrong here.)
2. The Adeptus Mechanicus stops being treated like a regular Imperial organisation as far as the rest of our actions go. It would instead be treated as if it were a separate polity as the other Nine Worlds despite inhabiting the same system, just like the Mechanicus in the Imperium.
PART B
Fabricator-General Scott fails at delegation in another way besides the Administratum reform. This second failure is one I consider to be more serious.
Avernus is not the extent of the Avernus system. In it there exists three gas giants of which one of them - Cumae - has multiple worthwhile moons with lots of mineral wealth on them.
Fabricator-General Scott's failure is in micromanaging them instead of letting others run them just like she lets other Fabricator-Generals run the Forges of other planets. Were she to stop doing that and were they allowed to focus inwards for a while, exponential growth would kick in and we would have at least one Forge World to provide a massive amount of materiel to the Imperial Trust. The concerns about such a location of strategic importance wouldn't matter because it would be located within the same solar system as an already existing Trust world, and more than that the Trust world in question is Avernus - home of the most hardcore regular human soldiers in the entire galaxy.
The Imperial Trust's industrial productivity would skyrocket because of it. Fabricator-General Scott's prestige and political capital would go way up, allowing her to pass whatever reforms she wanted much more easily. There would be no increased security risk thanks to where the Forge World would be located. A Forge World powered by technologies from the Dark Age would make the Imperial Trust nigh on invincible against the coming Ork threat. We are this close to replicating Callamus and ensuring our survival for millennia to come.
And yet because of her micromanagement, because she won't give the responsibilities to develop those moons to someone else the same way the Lord Sector gives the responsibility to develop planets to Planetary Governors, the speed at which they'll grow will be greatly limited.