Logophile is correct, we don't have a formal creed, in the same sense as the Westminster Confession of Faith or the Nicene Creed. But we do have official doctrine.
Yes, we do have official doctrine. And as you pointed out in your first post, it has remained remarkably consistent over the years.
One reason for this consistency is that the prophets of the Church have tended to confine their official pronouncements to essential matters related to the Gospel. Thus, they have often testified about Jesus Christ, the Atonement, the Restoration of the Gospel, and so forth. These are issues upon which Mormons tend to agree.
However, there are many issues on which the leaders of the Church have never taken an official position. For example, the LDS Church has no official doctrine on Evolution. This seem surprising to many people, Mormons and non-Mormons alike, especially since prominent Mormons have expressed strong opinions on the subject in the past. Nevertheless, the leaders of the Church have refrained from taking sides in the Evolution-versus-Creationism controversy.
Thus it is quite possible to find Mormons in good standing who believe in Young-Earth Creationism and others who believe in Darwinism.