Mormons do not believe Jesus was always God
I am not sure exactly what this means. Even if taken at face value for his meaning, the whole paragraph comes off incoherent. A better case would be to say that Mormons believe Jesus was always Divine, but never more than the Son of God. He has never been considered by Mormons as God the Highest. It can't be said Jesus has never been considered God the Father, as there are some instances in the Book of Mormon that he was given that status. In fact, later on in his argument McDermott brings up that idea as a way to prove Mormon inconsistencies or at least theological developments of the Godhead. As usual, perhaps because of his evangelical background, he simplifies what is a complication in order to score points.
Again, he has a great start with, "It is indeed, for it purports to give us another history of what Jesus said and did—not one to replace the Jesus of the gospels but to supplement that record." However, like the quote above, he becomes incoherent in the argument trying to shoehorn what he believes in with what Mormons believe and comes out sounding ignorant. If he would have left off with that sentence and then simply stated, but other Christians don't believe in the Book of Mormon, then the argument would at least make sense. Instead he goes off where such an argument has been weak for at least the last 150 years (from the start if you count Catholics). Other Christians in the first-century, even if you call them heretical, kept other writings as Scripture that Protestants have dismissed.
His main thesis is that the Bible is a collection of witnesses where the Book of Mormon has only a witness of itself. That is all fine for a simplistic interpretation of history and Biblical textualism. The problem is it too quickly dismisses the lengthy scholarly questions of authorship and authentic testimonials. Strangely enough, it is the multiplicity of testimonials mixed with inconsistencies in the texts that put the Bible to question for some. His idea that "the testimonies we have to the Palestinian Jesus date from the same century as [the Bible] Jesus," is not universally accepted. Even if taken as true, there is a huge question of what part of that century. No matter how much the testimonies of the New Testament matters, there is still no original texts in existence.
Besides, the Book of Mormon is considered a testimony of the Bible and Jesus Christ. It is to be a "Fourth Gospel" rather than a separate textual entity. To create a separation where one doesn't exist is to reject the purpose of its existence to bolster (and not take over) the Biblical testimony. Even if it purports to be ancient, it is considered a "Modern" Scripture for a reason; to warn us today that the prophets and prophecy of yesterday are still of relevance. There are, of course, other purposes of the Book of Mormon. All of them have been ignored and rejected by the simplistic world view of McDermott who can't see past his limited preconceptions enough to make an argument reaching beyond his own self-congratulatory audience.
Third, there are inconsistencies between the Palestinian Jesus and the American Jesus. For example, while the American Jesus promises the land of America to the new Israel as a “new Jerusalem” (3 Nephi 20:22, Ether 13:3), the Palestinian Jesus speaks only of a kingdom of God that is open to people of every land. His promise to the meek is that “they shall inherit the earth” (Matt. 5:5). His apostles write that Jesus’ followers still seek a country (Heb. 11:14) and “should be the heir of the world” (Rom. 4:13). People will bring into the New Jerusalem “the glory and honor” not of a single nation but of all the “nations” (Rev. 21:26). So the Palestinian Jesus seems to think of the coming Kingdom as a worldwide phenomenon not limited to one geographical part of the earth, while the American Jesus is fixated on America.
I quote the whole paragraph above because those who honestly understand the Book of Mormon and Mormonism quickly recognize the lack of research it had to take to come to that conclusion. If anything, the Book of Mormon is rather universalist in its declarations of worldwide salvation and Kingdom building. One only has to read the First Book of Nephi to see the interplay between the American Continent and the world. Just like Jerusalem for the Bible, America is a center place where the gospel spreads to other lands. Many times the Book of Mormon hints at or right out says that there are promised lands all over the place for different people. The heavy use of Isaiah quotes are to teach how widespread and important having promised lands are to making the whole Earth a Kingdom in the final days. Although not directly related to what is said in the Book of Mormon, he ignores Joseph Smith saying the Kingdom of God will fill North and South America, and indeed the Whole World. All of this is ironic considering the current argument of many Evangelical Christians that America is a Christian nation in much the same way that Palestine is a Jewish land.
His "other discrepancies" are that Jesus in the Book of Mormon praises the faith of the twelve in the America's, but criticizes the faith of the Twelve he chose in Palestine. Also, that he didn't allow anyone in Palestine to remain alive (I disagree with his interpretation of John 21:23. I take it to mean that Jesus didn’t say if John would not die, but that it is none of your business to know if he will live), but does that with three American disciples. I don't find these a discrepancy as much as a difference in time and place. In other words, because Jesus might have said or did something different in one place than in another, that makes the Book of Mormon a discrepancy. He simply dismisses the idea that those in the Americas were more faithful and righteous than those in Palestine and therefore more worthy of greater blessings. That is a discrepancy from McDermott’s early assertion that Jesus taught the universality of the Kingdom of God. Ironically it begs the question of Jesus the Christ as an historical figure. The idea is basically if it didn't happen in the Bible then it didn't happen.
I am going to leave the rest of the criticism to Summa Theologica that covers exactly what Mormons believe about Jesus and God. The comment section is also worth reading. The article is well enough done and in-depth that anything I would write would be a poor substitute.