The heavenly sanctuary doesn't need to be cleansed since sin can not dwell in the presence of God. The passage in Daniel might be talking about the earthly sanctuary, that is, the people of God, since Christians are the temple of God here on Earth.
The seventy weeks can't have been entirely fulfilled yet since two of the purposes stated in Daniel 9 are "to make an end of sins" and to establish "everlasting righteousness." These two things haven't happened yet.
The first 69 weeks took place from the command to restore Jerusalem to Jesus beginning His earthly ministry. The three and a half years of Jesus' earthly ministry was the first half of the seventieth week. Since the first half of the seventieth week represented a significant three and a half year period in time, the last half of this week must also represent a significant three and a half years in history. The last three and a half years represent the upcoming three and a half year tribulation period.
The last half of this week can't be the period from 538-1798 because we are still dealing with the seventy weeks which were representative of 490 years. Thus you can't isolate the last three and a half days of this prophecy as 1260 years.
The statement about 2300 days means that the tribulation period was originally to have been 2300 days but "except those days be shortened there should no flesh be saved", it was shortened to three and a half years.
1844 is not a significant year whatsoever as far as Biblical prophecy is concerned. No big event happened, nor were any significant scientific discoveries made to signify the time of the end.
The notion of the investigative judgment is totally false because the Bible clearly shows the opening of the books takes place after the millenial reign. Besides, it is just ridiculous to think it would take Jesus 168 years and more to go through the deeds of every righteous person who ever lived. God will probably take a fraction of a fraction of a second to go through the books of remembrance.
Basically what Seventh-day Adventism comes down to is the false prophet William Miller predicts the world will end in 1843. Before 1843 he changes the date to 1844, which just shows what a winner he was. The world doesn't end. Like all false prophets, however, Miller, with the help of the plagarist and possible Jewess Ellen G. White changes the meaning of the prophecy to say that it meant the start of the investigative judgment.