The sale of Song would force us to sign somebody of the same position, and considering we haven't sold Song yet, it's not surprising we haven't brought in M'Vila.
I'm not saying we definitely will, but your logic of 'If we were, we would've already' just isn't true.
I think Capoue is more likely signing if we do sell Song. M'vila's problems are too much for Wenger i think. Also I think Song's agent started all this talk of move to Barca so we'd offer better contract for him. By the looks of Song he looks happy with us.