There have been numerous so-called 'prophets' who have claimed heavenly visions, as well as a divine calling to be messengers of God.
True. Joseph Smith (LDS) being an infamous one, and yet he wouldn't be the first nor last. This is why God, in the scriptures, warned about false prophets that would be present, along with the true:
Mat_24:11 And many false prophets shall rise, and shall deceive many.
Mat_24:24 For there shall arise false Christs, and false prophets, and shall shew great signs and wonders; insomuch that, if it were possible, they shall deceive the very elect.
Mar_13:22 For false Christs and false prophets shall rise, and shall shew signs and wonders, to seduce, if it were possible, even the elect.
1Co_14:32 And the spirits of the prophets are subject to the prophets.
Eph_3:5 Which in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit;
Eph_4:11 And he gave some, apostles; and some, prophets; and some, evangelists; and some, pastors and teachers;
2Pe_2:1 But there were false prophets also among the people, even as there shall be false teachers among you, who privily shall bring in damnable heresies, even denying the Lord that bought them, and bring upon themselves swift destruction.
1Jn_4:1 Beloved, believe not every spirit, but try the spirits whether they are of God: because many false prophets are gone out into the world.
Their supporters all 'swore' by these 'prophets.'
Of course. Many believed Hananiah (Jeremiah 28:1-17) the false prophet in the days of Jeremiah the true prophet. The devil always raises up a (or many) counterfeit(-s) when the true is revealed and present. It was always been this way from the beginning. Even when Enoch and Noah were prophesying, others were raised up by the devil to counter their message, with messages of "peace, peace", and "peace and safety".
Their supporters were all sincere in their beliefs
I would say as a general rule, "No doubt, no doubt.", though some might be in it to simply make money, or for their own gains (say like the nearest to Benny Hinn, etc).
I am most sincere, but being sincere is not the basis for deciding what is true from false. The Bible (KJB) is the standard for delineating Truth from Error, not myself (or my sincerity).
However, when comparing all their 'revelations' it becomes quite obvious:
(1) Their visions and messages are disparate.
(2) They all inevitably contradict orthodox doctrine.
(3) Their followers all believe they alone hold divine truth.
(4) Their prophecies fail.
(5) Each sect has its own unique laws and commandments which must be kept, lest they perish.
Generally true, though not always in every case. Some prophecies can be realized even with a false prophet, as it is written in scripture:
Deu_13:2 And the sign or the wonder come to pass, whereof he spake unto thee, saying, Let us go after other gods, which thou hast not known, and let us serve them;
Moreover, for Seventh-day Adventists, we do not, and have never, claimed that we "alone hold divine truth". For instance, many (not all) Baptists hold to the literal, natural, consecutive 7 days of Creation by God about 6,000 years ago. Truth is not owned by us. Truth is God's, and He shares it with whom He wills. Another for instance, is in the teaching that Michael is just another designation for the Son of God, and this was (and in some cases still is) held by many, including Baptists, Methodists, Anglicans, Theologians, Scholars, etc. The truth of that matter is not merely held by us (Seventh-day Adventists). Thus goes for all the truths we hold (state of the dead, Justification, diet, etc), with the possible exceptions of the Investigative Judgment and Heavenly Sanctuary, which all are free to claim as truth also.
All true biblical prophets had the approval of God, as evidenced by the numerous miracles they performed.
Miracles are no sign of a true prophet, though they can be present with a true prophet. John the Baptist, as others I can list from scripture, did no miracle (like Daniel, etc). I can name false prophets which did do miracles.
Furthermore, in studying their doctrine it is obvious they do not contradict each other.
Truth, as sister White herself does not contradict herself, neither the previous prophets before her.
Thus, verification of their divine calling came from their miracles, which could only be possible by the power of God, and the harmony of their doctrine.
Not true, see Daniel, John the Baptist, etc. No miracles.
EGW fails the test of God as set forth in His Word.
I, of course by the evidence I have seen, disagree with you.
There is no doubt she was a prolific writer who had help from her many 'spirit guides,' 'literary assistants,' and large library.
Moses was guided by Jethro, and his assistant was Aaron, Miriam and also Joshua. Daniel had schooling, etc. The libraries of Israel I am sure were quite large, for Solomon, and others like Jeremiah (whose scribe and assistant was Baruch), and likewise for Paul (whose assistant was at least Tertius (for Romans)).. Daniel, Peter and Paul were all visited by angels, among others.
She was also adept at mesmerism
This is simply an accusation, baseless in evidence. It may be safely ignored. She wrote whole books against "mesmerism", see "Mind, Character and Personality", Volumes 1 & 2 to begin with.
which enabled her to use Svengali-like powers over the men.... a la Jim Jones.
Jim Jones had some interesting connections, and his basic background was in Roman Catholic social justice theology. He also had people kill themselves (at gun point no less).
Sister White is far removed from any such thing. To say otherwise, is to paint with a palette of colour representative of the enemy of all souls.
Thank you for your personal commentary. I disagree with much of it, based upon my scriptural & historical position evidenced in facts.