Well, people can be mistaken, delusional, crazy, whatever... it doesnt necessarily mean you are from the devil... everyone proclaims what he or she believes...
of course there is the option that she was one of those false prophets that Jesus has warned us about... could be...
another option might be that they were out for profit, they have realized that a lot of people followed Miller and seen how easy it is to get a followship... who knows... times were hard back in the day and in the end I think they made quite a profit... I am not implying anything just presenting possible options...
another option might be that she partially from God and partially from the devil, even Jesus rebuked Peter when he said something that was against the Spirit. Yet I wouldnt say that Peter was from the devil. In the end I do not know how this devil thing works and I dont know if I wanna know, so I will not pass judgment.
Like I said egw is somewhat behind, should the Lord ever wish to make me believe her writings are genuine, worthy to be acknowledged, or implemented into my lifestyle, He will let me know... thats how I deal with. As seen I have presented my doubts and issues with her which are the base for my not taking her word as inspired. I prayed on it, so its up to the Lord, as of yet, He hasnt showed me to follow her writings...