It seems the logic being used:
1) restricts answers to inadequate, mutually exclusive choices
2) misses the context, and
3) needs to carefully define terms.
It's necessary to define exactly what the law of "incest" is referring to.
Thus far it seems that the command against incest a) was not binding on Adam's immediate descendants; and b) is binding on all men today. From that (given the assumption that moral laws are binding on all men at all times, and that the ceremonial laws are not binding after Christ) we deduce that incest is neither a moral nor ceremonial law.
No,
The Westminster Larger Catechism states that incest is generally related to a broad application of the seventh commandment. Not the exact specification in Leviticus as it was to Israel, though. That was a civil law given Israel.
E.g.
Let's say speeding is prohibited in the Levitical Law, let' say a chariot cannot drive more than 25mph within one mile of the Temple.
Could that broadly be related to one of the ten commandments?
Yes, not acting in a way that preserves life (positive), or acting in a way that endangers life (negative). But is that 25mph? What about in a society that does not have a temple? What about one with different logistics around its temple? What about one that does not have chariots? Or one that does not have people slowly walking away from the temple on sabbath?
It would be a sin to violate the 25mph for the Israelite, but that would not exactly be transferrable to all nations and situations, though there is a moral principle (not behaving in such a reckless manner as to endanger life by driving too fast around public gathering places.
The general equity gets to that- it does not really restrict or eliminate the moral, ceremonial, or civil nature of the law.
Another point is context, is incest defined as relations among the immediate federal head of the human race?
The reasoning presented assumes that is, by definition, but I don't think incest even includes that- it's not a matter of God giving opposite commands, it is defining what you are talking about. For example, one could say killing is wrong, moral law, always has been- but does that include self defense?
Also, the answer must be put in context.
The application of the Ten Commandments became much more explicit in the New Testament.
Our Lord, in the Sermon on the Mount, included application of being "angry with your brother without a cause" as violative of the Sixth Commandment.
We understand the broad application of the Ten Commandments much more clearly in light of the New Testament- does that make sense?