In the sense that EGW did indeed speak for the Lord, I don't believe she did so 100% of the time. I would opine that Paul and Peter were also prophets. They were convinced that Jesus would return "soon" as in within their lifetimes. He didn't. Does that invalidate the fact that they spoke for the Lord? EGW was among those who were committed that Jesus' second coming would be in 1844. He didn't. Was her "prophecy" in error or was she not yet commissioned as a prophet? She prophesied that the RCC and papacy would be the end-time beast/antichrist in an era where it was fashionable to persecute the RCC. Here we are, over 100 years later; and I don't see bands of marauding Catholics running around the world beheading people and causing general chaos around the planet. There is another religion that is already doing that quite effectively.
I do believe the close of probation is a real deal. However, I believe it occurs immediately before the 7 bowls of God's wrath fall on the earth. I also believe probation closes because all of the "true believers" will have been translated off of the planet so they don't have to endure God's end-time wrath. Everyone who remains on the planet (at least in the Middle East and surrounding areas) will have chosen to reject Yahweh and worship the beast/antichrist; so in a sense, they will have closed their own probation.
"I believe" does not imply that I think those of you who have a different opinion are wrong - you just believe differently than I do about the events leading up to the second coming (like the timing of the close of probation). I also believe that prophecy is intentionally sufficiently vague that none of us really "know what's gonna happen until it starts to happen. It was so vague in the Old Testament that the spiritual powers of the time were unaware that if Jesus died and was resurrected, they would be doomed; and the victory of grace and mercy over evil, sin, and death would be certain. (See 1 Cor. 2:8). Eschatology could very well be equally as vague.