Hello again __________,
The significance of the temple of the tabernacle of the testimony being open in Heaven in 15:5 signifies a time frame: the time of the Day of Atonement. It was only during the DoA that anyone (specifically the high priest) would see that part of the temple. Yes, it is at the end of the DoA, but it is during that time when God sends out the seven Angels (the same time the four Angels will loose their holds on the winds in 7:1).
The “seven angels” appear 9 times in the KJV in the Revelation. Each time (in the trumpets of 8 -11, in Heaven during the time of the plagues in 15, on earth during the time of the plagues in 16, during the explanation of the judgment of the great whore in 17, and the blessed judgment of the bride of Christ in 21), it is a time of judgment. Perhaps they are dressed as ministers of the temple–priests–because they are carrying out God’s priestly judgment in the various stages before His final executive judgment of 20:9-15.