"Is Spouse A married to Spouse B? Yes."

This is where we part ways.  If you answer "yes" then everything else you say follows. But marriages can be dissolved. I'm not a theologian, but in my heart I don't really believe that a woman like the one described is still married to the first spouse. Maybe the first marriage was never valid, in which case yes she can seek an annulment. But, it's not adultery in that case - the first marriage never was. 

Or (and you'll disagree with me here), maybe the first marriage no longer exists. 

What would you say if in the first marriage neither partner was baptized at the time? Or, if only one partner was baptized? Can the marriage be dissolved? In both of these cases the church allows for the dissolution of the marriage, "for the sake of the faith". There was a valid marriage, and the church then dissolves it for the sake of a greater good.

I believe the church can and should extend this idea, as the Orthodox have.