The 1841 British edition replaced beheld with behold. The resulting text, “and behold that the words of the Lord had all been fulfilled”, doesn’t work. It would work if the subordinate conjunction that were also removed (“and behold the words of the Lord had all been fulfilled”). The 1841 change to behold is clearly a typo, not the result of editing, especially since there appears to have been no conscious editing for that edition. The 1849 LDS edition restored the correct beheld here in Ether 15:33.
Summary: Maintain in Ether 15:33 the past-tense form beheld, the reading of the earliest text (“and he went forth and beheld that the words of the Lord had all been fulfilled”).