Ok screwed up my initial plan so let's redo that.
Going to be honest here. I really do not like the idea of the Alpha Legion being the premier check to the Inquisition. I am very worried about a potential war of conspiracies that feeds on both sides worse traits. That the Alpha Legion descendants seem more likely to turn renegade or run some conspiracy that forces them out.
I'd propose we use the Ultramarines so the chapter can be under their aegis as well. Dark Angels were considered but I'd rather not provoke the Inquisition to investigate them. Using the Legendary Gene seed as well in the hopes we can get either some adaptability or their organizational knack.
I fully expect that some level of Inquisitional antagonism is inevitable and we should account for that in the planning. I doubt Alpha Legion would be fatal (outside of godawful rolls), but I don't particularly want to risk them either.
Homeworld fulfills the near terra mission and furthermore points towards anti ambush and there seems to be no major need for homeward investment that might interfere with their duties.
[X] Plan: Ultra Reliable Agents
-[X] The Ultramarines The most pure, 'baseline' of Space Marine Gene-seed the Ultramarines are a lineage that gives no great benefits but in turn does not have great downsides. They do seem to have started a minor tradition of maintaining ties with their descended chapters however.
-- [x] Legendary Gene-Seed
-[X] Ikkite: A forest planet notable for being covered in spiky, nearly indestructible planet life, the humans of this planet have learned to keep aware of their surroundings even when in the fiercest of combat if only to keep from being impaled. (Near Terra).