The Cornerstone of Adventism

In 1858 Ellen White published that God had laid a solid immovable platform of truth in the Advent movement and that God was the Master builder. See 1SG 169 or EW 258.

Since Jesus is the Truth and and no other foundation can be laid than Him (1 Cor 3:11) and He is the cornerstone of God's Church then how could the Adventist Church build a solid immovable platform when according to our church today, they believed the so called "heretical doctrine of Semi-arianism?" If our pioneers were wrong about the person of the Son of God, then the cornerstone of Adventism was not laid correctly. When God is the master builder and lays a cornerstone would he allow that stone to be laid wrongly? How could Ellen White be shown by God that the system of teaching held by Adventists were "solid" and immovable" if the central piece of the foundation was completely wrong?