I believe the author of “Kingdom of The Cults” removed SDA from his list of cults some time ago and gave the reasons why. You might want to check out the latest edition of that very well researched classic from your library if they have it.
Actually, Walter Martin never included Seventh Day Adventism as a cult. Although he considered some of their teachings as wrong and unbiblical and openly criticized teachings such as mandatory sabbath observance, the investigative judgment, etc., on the central doctrines of the faith (the Trinity, the virgin birth of Christ, the resurrection, etc.) he considered them to be right on.
He wrote a whole book on the SDA church called “The Truth about Seventh Day Adventism” which marked his divergence from those who call them a cult.
I worked for Martin’s organization, Christian Research Institute, for four years when I first got out of college in 1966. I knew him well.