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I've been "flamed" a few times here on Trulia.com for suggesting that EVERYONE buying a home engage the services of an Attorney. The reason I've been slammed was because it seems only here in New York do we use Attorneys. Because your situation is clearly a legal one you need to consult with your Attorney.
If you don't have one, get a recommendation to a good local real estate Attorney.
IMHO the Seller should be held accountable for the delay in closing; they clearly didn't have all their "ducks in a row" when they decided to sell this property and this is now inconveniencing you.
Builder contracts usually have a window of up to six months to deliver units completed. Some, depending on size of building, obtain temporary CO's and can close a floor at a time.
You should check with your attorney to get an update and new timeline of what still has to be done. It may not be just a CO, there may be items outstanding.
Hope that helps.