Perhaps "terraforming" an asteroid is simply erecting/installing habitation domes, or maybe hollowing portions out for the population that is placed there.

IDK, it's something that I personally think is a bit silly to overanalyze.

Presumably because said cities have been bombed to dust.

I think the problem you're going to find with trying to get your questions answered is that you're kind of overanalyzing (seemingly random aspects of) Sins.

I have no opinion at this time. TBH, I don't actually care about what is chasing the Vasari, as at present I'm more concerned with just having fun playing a game than trying to delve into the mysteries of the background.

That's not to say I don't want to know what is chasing the Vasari, but IMO, the hidden enemy of the Vasari is a bit like the 2 lost primarchs in Warhammer 40,000:

We don't know what they are or what happened to them, and we don't want to know even if we say we do.

In the same way, we really don't want to know what's chasing the Vasari, as it would probably detract from the mystery and interest of the backdrop that Sins is set against.

That's not to say that if the Devs came forward and said "this is what's chasing the Vasari", everybody would be livid with nerdrage. After all, if the Devs did do so, then we could probably expect something in the future of Sins that expanded upon said threat.