We believe there is no difference between a prophet called before the days of Christ and one that is called during or after the days of Christ. They all have the gift of prophesy and are able to receive prophesy from the Lord. We believe that all who are called as apostles are also prophets, seers, and revelators. An example of a prophet that came after Christ was the apostle Paul who predicted the apostasy.This is why I believe Jesus is the final word for these last days. Hebrews 1:1-2.
God’s Final Word: His Son
1 In the past God spoke to our ancestors through the prophets at many times and in various ways, 2 but in these last days he has spoken to us by his Son, whom he appointed heir of all things, and through whom also he made the universe.
The scriptures in James isn't referring to salvation, but evidence of salvation. We are saved by faith in Jesus, then good works naturally follow. All scripture has to be taken in context with other scripture. The problem the cults have is they use one scripture that agrees with their doctrine to attempt to cancel out another scripture that doesn't line up with their doctrine. That's not scholarly.
This is very good explanation of the difference between Old Testament and New Testament prophets.
Prophets: New Testament vs Old Testament | Fountain of Life
To protect yourself from abuse, it's important to understand the office of the New Testament prophet and how this gift DIFFERS from the Old Testament prophet.www.fountainoflifetm.com
Another problem I have is with polygamy. According to your Covenants book polygamy is an everlasting Covenant. Yet, now it isn't practiced. This is interesting.
2 Thessalonians 2:2-4
2 That ye be not soon shaken in mind, or be troubled, neither by spirit, nor by word, nor by letter as from us, as that the day of Christ is at hand.
3 Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition;
4 Who opposeth and exalteth himself above all that is called God, or that is worshipped; so that he as God sitteth in the temple of God, shewing himself that he is God.
In this verse, Paul tells us that before the second coming of the Lord, there would be a falling away first. The term "falling away" comes from the Greek word, "apostasia" which is apostasy. Paul also said the following:
2 Timothy 4:3-4
3 For the time will come when they will not endure sound doctrine; but after their own lusts shall they heap to themselves teachers, having itching ears;
4 And they shall turn away their ears from the truth, and shall be turned unto fables.
Thus we believe that Paul was a true prophet the same as any prophet in the Old Testament. When God calls one to be a prophet, they are given the spirit of prophesy and the prophesies they prophesy are every bit as true as any given in Old Testament times.
Regarding your question on polygamy. we believe that God at times may command his people to live the law of plural marriage and at other times he may command us not to live it. The New and Everlasting Covenant of Marriage is to enter into an everlasting covenant of marriage. This can entail marrying one person of the opposite sex or many if the Lord commands it. You are not required to marry more than one wife unless it is commanded by the Lord. We believe that Joseph Smith was commanded by the Lord to live the law of plural marriage by the Lord and latter on in our history we were later commanded not to live it by our prophet Wilford Woodruff. In ancient times, Abraham, Jacob, David, and Solomon were allowed to marry many wives. In the case of Abraham and Jacob, all their wives were acceptable before the Lord. In the case of David, all his wives except Bathsheba were acceptable before the Lord and in the case of Solomon, many of the strange wives he took were offensive to the Lord. You can read the revelation received by the prophet Joseph Smith in Doctrine and Covenants 132. You can also read about the revocation of plural marriage in the Official Declaration 1. Plural Marriage is not a requirement to be exalted into the kingdom of God unless God commands it. The New and Everlasting Covenant of Marriage is nothing more than being sealed up to eternal marriage through the priesthood of God and is performed in our Holy Temples.