There's been no official announcement about Halo Wars DLC yet.

Most of you guys are on generally the right track.  :) There were Flood infestation on many of the Forerunner artifacts, including the Halos themselves and the shield world visited by Spirit of Fire in Halo Wars.

During the events that take place during the game, they were pretty busy.  :)

A fully-upgraded Covenant leader and a Scarab backed up by a bunch of Engineers is a pretty formidable force.

Automatic difficulty can get well above the "Legendary" difficulty setting, so keep playing.  :)

We were very happy with the way co-op turned out.  I'd suggest trying it, even if you think you won't like it, though I agree that a good partner makes all the difference.  :)  Since "player attention" is like a hidden resource, co-op play should only make the campaign easier if you and your partner ...

Spirit of Fire is really, really, really far away from Earth when the story ends, and the ship is severely damaged.  It's safe to assume that phoning home is not an option for the crew.

I imagine Grunts would be perfectly happy if they didn't have to fight humans any more.  :)

There is in fact a Halo Wars strategy guide.  As a gamer who likes strategy guides and has an enormous stack of them at home, I can tell you that it's pretty good.  :)

Timeline events are also unlocked by playing skirmish games on different maps using different leaders (both Covenant and UNSC).