Most Christians believe in pre-incarnational appearances of Christ, including some called the angel of the Lord, but wouldn’t agree necessarily to identify the angel Michael
as having been one of these. Like your other mention about a detail concerning sin and the devil, these are internal theological matters. Good Christians can differ on details or on the identification of vaguely defined biblical personages. The 7DA are typically quite bold about how to live a Christian life, mistakes or not.
No, these are not. If the SDAs say Jesus is an Angel, just an angel, even the highest one,that is saying that He is not God
That puts the group in the same category as it's sister religion, the Jehovah's Witnesses (they grow up out of the same mileu)
Christianity at it's basics is: There is One God, the Father is God, Jesus is God, The Holy Spirit is God. Jesus is 100% man and 100% divine. He died for our sins. Without His sacrifice we could not be saved by our own efforts.
That's it in a nutshell.
If one says that Jesus is not God, either just a holy man (Jehovah's Witnesses) or a super-angel (SDA), that is not Christian.
Neither is saying there is more than 1 God -- Mormonism -- that instantly cuts off the continuity with Judaism.
Neither is saying that Jesus was only Spirit or only man, either way negates the incarnation and eliminates the purpose of the incarnation and sacrifice as does the last (Jainism -- not even Pelagianism which acknowledges that the sacrifice was necessary)