I've got the solution at least to the ground attack problem...
The alien population sits on Sirius A III - Moon 10, while our forces are on Sirius A III - Moon 7.
Wait, you mean the Moon 10 aliens with a thermal signature of 5, right? And a single ship? That looks, at the first glance, like a precursor outpost. From landing troops there in my copy, it's only a single DSTS.