The 1852 LDS edition replaced the conjunction for with and. This substitution appears to be accidental. The original for was restored to the LDS text in the 1920 edition, undoubtedly by reference to one of the early editions. The for is necessary because it explains that the dissenters became more numerous as a result of their effective (but deceitful) proselyting methods.
Summary: Maintain the conjunction for in Mosiah 26:6, which is used to explain why the dissenters were so successful in gaining adherents.