Mold on drywall. Should I clean-up or replace?
General practice is going 6 inches above molded area and cut it out.  You must inspect the backside of the drywall.  If you can't you need to keep cutting out until you hit 6 inches of unaffected wall.  You then need to follow a mold remediation plan - not sure how anything else in the wall looks but useless replacing the drywall if you will put up new drywall on moldy framing.  If there is any humidity/dampness it will continue to grow.  

As far as a barrier... I guess you could put up plastic.  Not sure how affective it would be because water will find a way to go somewhere.  Where do you want leaks to go?