(Revelation 5:10) - and you made them to the God of us kingdom and priests, and they are reigning upon the earth. (NWT Kindom Interlinear)
(Revelation 5:10) . . .and you made them to be a kingdom and priests to our God, and they are to rule as kings over the earth.” (RNWT)
The original Greek says certain ones will be reigning, (βασιλεύουσιν) it does not say they will be kings.
Also, the original Greek clearly says that they will be upon the earth - not above or over it.
Scripture proves scripture: The messianic prophecy at Psalm 45:16 says, "You will appoint them as princes (not kings) in all the earth."
Princes (rulers, chieftains, heads, officials under king) on the earth - not kings in heaven! Example: Daniel, Shadrach, Meshach, & Abednego were all rulers - not kings.
As for the cult's alleged 144,000 king-priests, Paul must have been exaggerating Christ's parallel to Melchizedek - if 144,000 window-cleaners etc. are also going to become king-priest Melchizedek clones:
(Hebrews 5:5, 6) "So, too, the Christ did not glorify himself by becoming a high priest, but was glorified by the One who said to him: “You (not 144,000 others) are a priest forever in the manner of Melchizedek.”
(Hebrews 7:11) "...what further need would there be for another priest (not 144,000) to arise who is said to be in the manner of Melchizedek." (not 'are' like Melchizedek)
(Hebrews 7:15) "And this becomes even clearer when another priest (not 144,000) arises who is like Melchizedek," (not 'are' like Melchizedek)