The following information persuades me Section 132 came through Joseph Smith and was reduced to writing on July 12, 1843:
The Nauvoo Diaries of William Clayton were written chronologically and have the following entries (exactly as in original):
In addition to the foregoing, I checked surrounding public events, and the diary is consistent with other records of those days. For example, the event on July 16th is recorded as having taken place "At Stand in Grove, West of Temple" and appears in a letter of Willard Richards to Brigham Young, the Joseph Smith diary kept by Willard Richards, the Levi Richards Diary and the Willard Richards Diary. The afternoon of the 16th also records a public meeting on the "Temple Stand" in the Franklin Richards, William Clayton, and Levi Richards diaries and in the Letter of Willard Richards to Brigham Young, as well as in the Joseph Smith diary kept by Willard Richards.
Disputes after Joseph's death also confirm a disagreement between Emma and the church over ownership in the Steamboat the Maid of Iowa.
These entries seem credible, and therefore I believe they show Section 132 was recorded on July 12, 1843 and originated from Joseph Smith. In addition, the August 12, 1843 meeting of the Nauvoo High Council records there was "teaching by Hyrum Smith" which four witnesses later confirmed included reading Section 132. These witnesses were Austin Cowles (who rejected the doctrine and left the church), David Fulmer, Thomas Grover, James Allred and Aaron Johnson. Hosea Stout was absent when Hyrum read the document, but was later told about the revelation. When Section 132 became public, Hosea Stout confirmed it "corresponded to what" he was told about the reading in August 1843.
It is possible to believe it a fabrication of Brigham Young. It was not made public until the 1850's, and the public disclosure was on Brigham Young's watch. But the document came into existence while Joseph was church president, and came through him. As much as a person may wish the document did not originate with Joseph Smith, the evidence appears to be more than adequate to show it did. It came from Joseph and was reduced to written form in July 1843.