How does the LDS Church defend the racist doctrines of its past?

OP quote is from the journal of discourses.

The Journal of Discourses is not an official publication of The Church of Jesus Christ of Latter-day Saints. It is a compilation of sermons and other materials from the early years of the Church, which were transcribed and then published. It included some doctrinal instruction but also practical teaching, some of which is speculative in nature and some of which is only of historical interest.

About the authoritative nature of said document:

Doctrinally, members of the Church were growing and learning. Most adults were converts who had to unlearn and relearn many doctrines. They were learning things which our children learn in Primary and Sunday School. Remarks were frequently impromptu. Close, friendly audiences frequently invited informal discussion of varied topics. There was occasional speculation about doctrines which have since been determined unimportant or even misleading.

Before diving too much into the Church of Jesus Christ of Latter-day Saint's past, I'd like to mention some core beliefs:

• continued revelation¹
• natural man is enemy to God ²
• repentance³

Repentance implies that a person turns away from evil and turns his heart and will to God, submitting to God’s commandments and desires and forsaking sin. All accountable persons have sinned and must repent in order to progress toward salvation.

As noted in the second quote members of the Church were growing and learning. Most adults were converts who had to unlearn and relearn many doctrines. They were learning things which our children learn in Primary and Sunday School. Revelation did not stop with Joseph Smith, the LDS believe it continues to this day.

For example in 1978 Official Declaration 2 the LDS church changed it's policy to allow any worthy male church member to hold the priesthood.

Accordingly, all worthy male members of the Church may be ordained to the priesthood without regard for race or color.

Opinion: I personally view this as proof of continuing revelation, one of repentance, and progress. We have all made mistakes in our pasts, even if we were striving to do what was right at the time. Reflecting on the past is ok, but we shouldn't let past mistakes overshadow where we are, what we have overcome, and what we are striving to become.

Specifically on progress/repentance:

We believe that God is the Father of all humankind and that “he denieth none that come unto him, [both] black and white … ; and all are alike unto God”Because all people are equal before God, we believe that no person can rightly claim to be superior to another because of their racial or ethnic background. In addition, we believe that it is sinful to look down upon, denigrate, or discriminate against others because of their race.⁴

From Race and the Priesthood I see the closest thing to a defense of past actions in regards to racism:

The Church was established in 1830, during an era of great racial division in the United States. At the time, many people of African descent lived in slavery, and racial distinctions and prejudice were not just common but customary among white Americans. Those realities, though unfamiliar and disturbing today, influenced all aspects of people’s lives, including their religion. Many Christian churches of that era, for instance, were segregated along racial lines. From the beginnings of the Church, people of every race and ethnicity could be baptized and received as members. Toward the end of his life, Church founder Joseph Smith openly opposed slavery. There has never been a Churchwide policy of segregated congregations.

During the first two decades of the Church’s existence, a few black men were ordained to the priesthood. One of these men, Elijah Abel, also participated in temple ceremonies in Kirtland, Ohio, and was later baptized as proxy for deceased relatives in Nauvoo, Illinois. There is no reliable evidence that any black men were denied the priesthood during Joseph Smith’s lifetime. In a private Church council three years after Joseph Smith’s death, Brigham Young praised Q. Walker Lewis, a black man who had been ordained to the priesthood, saying, “We have one of the best Elders, an African.”

In 1852, President Brigham Young publicly announced that men of black African descent could no longer be ordained to the priesthood, though thereafter blacks continued to join the Church through baptism and receiving the gift of the Holy Ghost. Following the death of Brigham Young, subsequent Church presidents restricted blacks from receiving the temple endowment or being married in the temple. Over time, Church leaders and members advanced many theories to explain the priesthood and temple restrictions. None of these explanations is accepted today as the official doctrine of the Church.

_{all emphasis mine}

_{1 Modern Prophets and Continuing Revelation}

_{2 Mosiah 3:19}

_{3 Guide to Scriptures: Repentance}

_{4 What is the Church's stance on Racism, see also No More Strangers}