JOEL - I don't know why Brigham Young said that about the Nauvoo Temple being "nearly completed". I have explained elsewhere about the inaccuracy and unreliability of things reported in the Journal of Discourses.
Perhaps not every minor detail of the Nauvoo temple was finished as planned, but it was completed well enough to be dedicated so the saints could perform the temple ordinances, which is of course the main reason for having the temple.
After the baptismal font in the temple was completed on November 21, 1841, baptisms for the dead were performed there (HC 4:454) for more than five years before the temple was destroyed(see D&C 124 verse 33).
The capstone of the temple was in place by May 1845, and the edifice was ready for ordinance work by December of that year. Formal dedicatory services were conducted on April 30 and May 1, 1846. Once a temple is dedicated it is considered completed. Eventually nearly 6,000 men and women performed temple ordinances for both the living and dead before leaving for the West.
The temple served its purpose for those living in Nauvoo, thus fullfilling the commandment given to them of God to build it in the amount of time He gave them to build it.
During a Sabbath service, Orson Hyde explained why the Saints were compelled to finish the Temple, "If we move forward and finished this house we should be received and accepted as a church with our dead, but if not we should be rejected with our dead(Orson is refering to the same scriptures you mentioned, D&C 124: 32-33). These things have inspired and stimulated us to action in the finishing of it which through the blessing of god we have been enabled to accomplish and prepare it for dedication." Wilford Woodruff concluded the meeting, "The Saints had labored faithfully and finished the temple and were now received as a Church with our dead. This is glory enough for building the temple and thousands of the Saints have received their endowment in it." (Wilford Woodruff, Journal, 8 May 1846, CA.)