(a) There would not be such great agreement on its meaning.
Except there isn't any such "great agreement". In actual fact, cultures can and do disagree on morality, and in many respects they disagree quite strongly. The agreement you talk about is only among people from the same (or at least related) cultures, which proves nothing.
(b) No real moral disagreements would ever have occurred, each person being right from his own moral perspective.
This is purely specious reasoning. People who think they're right almost always try to force that viewpoint onto other people who disagree, thus generating the moral disagreements that this apologetic claims couldn't have occurred.
(c) No moral judgment would ever have been wrong, each being subjectively right.
Specious again. In actual fact, people who think they're right about something will argue against anything that contradicts them, even if there's facts and evidence which negates their own position and supports the other side. This is as true in morality as in anything else.
(d) No ethical question could ever be discussed, there being no objective meaning to any ethical terms.
Incorrect. All that's necessary for such a discussion is that there be a common meaning. It doesn't have to be an objective one.
(e) Contradictory views would both be right, since opposites could be equally correct.
And yet we see this every day, people holding contradictory views, both of whom claim to be right. But what usually happens is that you get enough people agreeing that one view is right, and that one is held as the correct one. If it turns out that there are problems with it, it's revisited later on.
As near as I can tell, the other apologetics you cited are little more than restatements of these five points. In short, your argument from apologetics fails miserably. because it assumes that its premise is true and never seriously considers the possibility that it might be false. In actual fact, it is trivially easy to point out the weaknesses of the apologetics you chose to use - I focused on the first five to keep this post from becoming too long, but rest assured that it would not be difficult to point out the same weaknesses in the other apologetics, since they're all the same at heart.