OK, Mo_, then we agree. As written, your syllogism is formally invalid, i.e. the conclusion does not follow from the stated premises.
Additional premises can certainly be added to make it work. Changed as you suggested, the argument is this:
P1: If and only if morality is objective, we can consistently have productive discussions with other people/cultures and speak meaningfully (without talking past each other). We can even criticize each other, legitimately. We can do things we should be able to do, like reflect on our past, claim to have grown, etc.
P2: In fact, we do these things anyways.
C: Morality is objective.
This strikes me as a weak argument. One can reject the idea that we can legimately criticize others’ conduct; moral relativists would disagree that such criticism is legitimate, as would anyone with any theory of a subjective morality. In addition, the initially unstated premise that being able to do these things entails that morality is objective (If C then A, the other half of the biconditional) also seems suspect. It’s quite possible that it’s best to treat morality as though it’s objective, even if it is in fact subjective, e.g. because it only really matters that everyone agrees on morality and treating it as objective encourages broad agreement.
Phyllo, I encourage you to read my exchange with Flannel Jesus in this thread, as I am challenging exactly this notion. Here is another example that I think makes a strong case for top-level objectivity with significant underlying subjectivity:
Language is at base subjective. If I decide to myself to call a rock a shporkle, that word will have subjective meaning for me, but when I ask someone to pass me a shporkle, no meaning will be exchanged. And yet, at the highest level, when the language is examined across all of its speakers, we can and do make objective statements about whether or not someone is using or pronouncing a word incorrectly. There is an objectively true meaning to words that emerges despite the subjectivity of any individual’s use of that lanuage. There’s fuzziness and error rates, but it would be hard to argue that the english language doesn’t exist objectively.