I am SDA so my views may be considered biased - and of course I accept the statement that Ellen White is an example of a true prophet, someone who had the real 1 Cor 12 gift of prophecy also mentioned in 1 Cor 14 and Eph 4..
In any case the Bible does speak about prophets and the gift of prophecy.
A good example of a major prophetic statement by Ellen White can be found at this link Chapter 40 of the book Great Controversy --
The Great Controversy
As SDAs we always say
all doctrine is to stand or fall "sola scriptura" and the Bible says that all prophets must be tested against well established doctrine that has been tested sola scriptura as we see in Acts 17:11 and in Gal 1:6-9.
Whether one is talking about Ellen White or not does not matter when it comes to what the Bible teaches about the gift of prophecy. The Bible does not say "the gift of prophecy works like this and this... unless the prophet is Ellen White".
So we have 1 Cor 12 for the gift of prophecy and we have Numbers 12:
Numbers 12:6 He said,
“Hear now My words:
If there is a prophet among you,
I, the Lord, shall make Myself known to him in a vision.
I shall speak with him in a dream.
Under the Theocracy of Israel false prophets were to be stoned and the reason for that is not "well if you have an incorrect idea in your head you should be stoned" - but rather it was based on the fact that the message from God to that prophet is infallible since God is the source and not the prophet. So anyone who claims they got a direct message from God and that message contradicts the Bible or is proven false then that is proof that they are a false prophet.
Of course Jonah was not a false prophet even though his prediction did not come true. And that is because of the Jeremiah 18 principle of conditional prophecy. Jonah does not specify 'The condition' in his prophecy -- but it was there all the same.