The original manuscript had an ampersand before therefore, but it appears to have been erased. The ultraviolet photographs clearly show smearing from the erasure, even up into the supralinear space above the ampersand. There is nothing wrong with “and therefore”; it occurs 18 times in the text. So the removal of the and here in Alma 37:28 seems to be a correction to what Joseph Smith actually dictated. For an opposite example where the scribe omitted the and before therefore, see under Alma 12:10.
Summary: Accept in Alma 37:28 the corrected reading without the and before therefore; Oliver Cowdery initially wrote an ampersand before therefore but then immediately erased it.