Yes, he was separated, and he told them that God told him to separate. Yes, FAIR is fair about this, and they are correct. I believe that you are reading too much into his statements, especially considering that he was an apostate. What if Judas said; believe me just as I am telling you that Christ did miracles and healed people, I am also telling you that He is not the messiah and he never freed us from the Roman yoke? What if Judas said; God revealed to me that Jesus was his son, however, He was not the messiah. I can tell you what I would believe. I would believe (just as in this situation) that maybe God did reveal to Judas that Christ was his son, but I would also believe that Judas, being an apostate, added the part that Christ was not the messiah based on his own belief and apostasy! Again, what about the other apostles, and again, what about the other witnesses? I wonder why God didn't tell them to leave the church!
First, FAIR is not being fair. Contrary to the idea proposed by the FAIR wiki, the physical separation from the Saints happened BEFORE David Whitmer said God spoke to him. This has been explained to you several times in this thread.
Second, you are begging the question. To establish that David Whitmer was apostate, you need to articulate why we should not take him at his word that God gave him authority. You have not done so.