"What is the relation of this verse to the work in the sanctuary? Is the work ongoing? Are we still in the time of 'probation'? How do we understand this verse?"
To take the last question first, we understand this verse (Dan. 12:1) by permitting the Bible to interpret itself. This is done primarily by way of Bible reaearch, and sometimes with the assistance of the spirit of prophecy writings of Ellen G. White.
"And at that time shall Michael stand up, the great prince which standeth for the children of thy people:..."
To add to what already has been said, most Seventh-day Adventists will concur that Michael is another name for Christ because there is ONLY ONE PRINCE who stands for the people. Rev. 12:7 tells us: "And there was war in heaven: Michael and his angels fought against the dragon; and the dragon fought and his angels." There are only two forces that command angels: the god of this world called Satan (v. 9), and the Son of man "who shall come in the glory of his father with his angels" (Matt. 16:27).
When Christ stands up, His mediatoral work in the Most Holy Place of the Heavenly Sanctuary will have ended. He lays aside His priestly garments and dons His kingly apparel as He proclaims: " He that is unjust, let him be unjust still: and he which is filthy, let him be filthy still: and he that is righteous, let him be righteous still: and he that is holy, let him be holy still" (Rev. 22:11) as He prepares to return to this earth to gather His redeemed (GC, p. 613).
"...and there shall be a time of trouble, such as never was since there was a nation...."
When Christ stands up, probation will have closed for the inhabitants of the earth, and there shall come a time of trouble such as never was, meaning calamity like this has never happened before. I believe the Seven Last Plagues of Revelation 16 shall occur at this time. Ellen White tells us that the people of God will not be immune to the discomfort and distress that shall take place during this time. She states that: "The people of God will then be plunged into those scenes of affliction and distress described by the prophet as the time of Jacob's trouble." Jeremiah 30:5-7. (GC, p. 616). BUT THE GOOD NEWS IS...!
"...and at that time thy people shall be delivered, everyone that shall be found written in the book."
Isn't that good news? God IS NOT going to leave His people to perish at the hands of evil men and women during this time of trouble. The Bible tells us that EVERYONE shall be delivered whose name shall be found written in the Book of Life!
THIS is the GOOD NEWS today just prior to that time to those who are being persecuted, misused, abused, and suffering at the hands of those who are led by the enemy. The word is: HOLD ON, just a little while longer. GOD SHALL NOT LEAVE HIS PEOPLE! Everyone shall be delivered, "everyone that shall be found written in the book."
M.A.