Someone mentioned the idea of having the Sisters of Battle stationed off of the Forge World, and while I like that idea, I'm a bit iffy on it as well.
While having a capable force defending our resources without using our own manpower is a good thing, that also means that that resource is split between us and them. They'll need gear and maintenance, too. But it still sounds like a good idea that I would vote for.
Until we get some word about that, though, I approve of sending some Marines to the Forge World. A world as important as that would be far too insulted by Chapter Serfs, whether or not they're capable. And it's not without precedent, leaving Marines to defend valuable assets. Imperial Fists and Ultramarines are most known for it, but even the Space Wolves give Marines to their Navigator House and certain planets important to them.
As for the Mantis Warriors, well . . . they finished a penance crusade. They *finished* a *penance crusade*. Meaning they went through the harshest examinations possible by the Inquisition and were deemed holy in the Emperor's light.
So if they want to position themselves in a nearby sub-sector, hell. Good for them and us. Having an entire Chapter as an ally would be amazingly good for us.
As for the newest part, I'unno. Just keep going, good Emprah.