The SA were NOT a rival. Were never perceived as a rival. An 'opportunity', to build a basically reformist mass organisation in which a revolutionary minority could have influence? They believed so.
Even if you accept the Machiavellian argument, and I don't, would you want to control and organisation bigger than yourself, or smaller than yourself? Obviously the latter. Therefore all their actions in the SA, were what they believed would deliver this prize, a mass organisation.
PS. Does anybody have any figures on how many members the Socialist Alliance had before SW joined it?