There are many conjectures against why this "obelisk" grave stone is on the White grave, but in essense it means little. First if one would do some research into grave stones, you will find that it was a very popular type of grave marker during the turn of the century.
Secondly, as many of her detractors forget, she was human. She was not God, therefore she was also susceptible to error, just like those men and women of the Bible. What I find so funny, is that doubters of her prophectic gift always search for these "errors" in her writings or person to disprove her, yet none argue against Peter for the errors he was spreading that Paul corrected him in. Or maybe we should discount all that Solomon wrote because he fell away from the Lord for a while. People are human and are not infalllible. Unfortunately, there are some SDA's that may seem to hold her up as infallible, just like those of the Catholic church hold the Pope as infallible.
Now for a personal story of my own. When my first wife died, I had to suddenly make decisions that neither one of us had ever talked through before. So, I decided to have her cremated and thought nothing of it. A couple years later, my grandfather's brother died. His sisters had him cremated, and my grandfather (Christian but not SDA) was beside himself and very upset with them. He said that it was a pagan custom to burn the dead. I had never really given it much thought until he mentioned that. Now, having done that to my wife, does that make me pagan? Of course not. I did, what I felt would be according to her wishes and to honor the end of her life. Now having spoken with my grandfather about this, I can't go back and uncremate her can I?
In essense symbols only ever mean what the person using them intends. Did Sister White intend for her grave stone to show that she worshipped Satan? Or was is to just mark the spot of her burial? I think by looking at the life she led and the things she wrote point us to the latter. Please keep in mind, just because God reveals things to certain people that no longer takes away their humanity in everyday life. We all make decisions based on circumstances that come up and don't necessarily look for God to give us direct answers to every one of them. God does not tell me daily what to have for breakfast when I go to my kitchen. He does not pick out my clothes or reveal to me what shampoo to buy when at the store. So why is it hard to fathom that God did not give Sister White every word or thought or action that she did. She was not some puppet that He controlled and personally led her life for her. She was human, just like Paul, Elijah, Moses, John the Baptist, and Daniel. Sometimes she wrote letters to others that were of her own personal thoughts, just as Paul wrote letters mentioned in the Bible that we have never found. God revealed things to her if and when He saw fit, and then she continued in life just like the rest of us do; trying our best to follow God through the leading of His Holy Spirit in all of our human imperfections. If she mentions that she was show "in vision" then we can know that she spoke of things God revealed propheticly to her. But also, saying "I was shown" could also mean through personal study and prayer. God shows me things every day, and so far none have required that I have the prophetic gift.
Now, as for symbolism in the Adventist church. I can say from personal experience that not all who attend church, even leaders, are followers of God. I was in the church for 10 years, and sometimes in leadership roles, and never really had a relationship with God. So, if I can say that about myself, I know that I cannot be alone. And I think that goes for each and every denomination our there, not just SDA. And I can be fairly certain that it can extend into higher leadership just by looking at all the corruption going on in American churches lately. Pastors, priests, bishops, reverends involved in sex scandals, money laundering, pornography, and even murders. So to think that Adventism is in some way immune would be naive. People are saved on an individual basis, not corporately. So, we must not forget that. Now having said that, why is noone accusing the Baptists or Presbyterians of being masonic and Satanic because of the symbols represented on their churches? The cross, yes it brings in remembrance of what Christ did for us, was also first a symbol of Roman punishment. Why honor the cross symbol when it was the means that put our Savior to death. We don't place large rocks in churches in honor of Christ being our Rock. Sometimes things are placed in churches because they are artistic or sometimes they distinguist the building from other buildings in the area. A steeple with a cross on it for example does not bring to mind that mason's or Satanist gather there. Most people see it and recognize it as a Christian church. And then therefore go there to find Jesus. Sometimes things really mean just what they mean and not have some secretive meaning behind them. I think if you ask the average everyday worshipper of a Baptist church why they have the symbols they have, none will tell you they do it in honor of Satan but want to put on the facade of Christianity. So I would say don't get too held up on the outward symbols and look on the inside to see if Christ is really with the individuals.