No, she didn't. A messenger yes, a prophet, no. She refused any title that would tend to remove Jesus from anyone's focus.
Her theology? Are you suggesting she made it up herself, and was the first to teach soul sleep? Ever heard of Martin Luther?
" We should learn to view our death in the right light, so that we need not become alarmed on account of it, as unbelief does; because in Christ it is indeed not death, but a fine, sweet and brief sleep, which brings us release from this vale of tears, from sin and from the fear and extremity of real death and from all the misfortunes of this life, and we shall be secure and without care, rest sweetly and gently for a brief moment, as on a sofa, until the time when He shall awaken us together with all His dear children to His eternal glory and joy...For since we call it a sleep, we know that we shall not remain in it, but be again awakened and live, and that the time during which we sleep, shall seem no longer than if we had just fallen asleep... Hence we shall censure ourselves that we were surprised or alarmed at such a sleep in the hour of death, and suddenly come alive out of the grave and from decomposition, and entirely well, fresh, with a pure, clear, glorified life, meet our Lord and savior Jesus Christ in the clouds...Scripture everywhere affords such consolation, which speaks of the death of the saints, as if they fell asleep and were gathered to their fathers, that is had overcome death through this faith and comfort in Christ, and awaited the resurrection, together with the saints who preceded them in death." Martin Luther
Bible...
His breath goeth forth, he returneth to his earth; in that very day his thoughts perish (Psalm 146:4).
For the living know that they shall die: but the dead know not any thing, neither have they any more a reward; for the memory of them is forgotten. Also their love, and their hatred, and their envy, is now perished; neither have they any more a portion for ever in any thing that is done under the sun (Ecclesiastes 9:5-6).
For in death there is no remembrance of You (Psalm 6:5 NKJV).
The dead praise not the LORD, neither any that go down into silence (Psalm 115:17).
So man lieth down, and riseth not: till the heavens be no more, they shall not awake, nor be raised out of their sleep (Job 14:12).
So no, Ellen White didn't invent 'soul sleep', for want of a better expression, nor was she the first to teach it.