Will the DLC packs, Caravan and Merc ... etc. ... will be available through Windows Live? I am not sure if Steam and Windows Live are the same thing. :facepalm:

Steam and Windows Live are not the same things. Activation of the pre-order DLC will occur through Steam unless I am mistaken.

do you mean will they be available for download without having to pre-order. if so, the answer is no

Available as in to buy separately instead of getting them as a pre-order bonus? There hasn't been any confirmation about it or not (that I've heard of), but I really doubt they'd make them available to buy separately.

And no, Steam and Games for Windows Live aren't the same thing.