If the ants had already evolved the sterile worker-caste system

Isn't this where we started from? We are talking about "phenotypes that are never expressed in reproductive individuals" so sterile workers must already be in the picture..

Ok so with C=0, the equation basically says "a trait can develop if it has a positive influence on the queen". Duh, that's a triviality.

Note that you can make an equation like "rB > C" always hold in any case by just making up values for B or C; I don't see the insight that gives you in cases like this where it resolves to a triviality like "B > 0".

More specifically, I don't understand how this relates to:

'Colony selection' and 'superorganisms' don't do the trick. You have to talk about shared genes in individuals, with conditional phenotypic expression.

I don't see what insight brought by Hamilton's rule "does the trick". It seems we are simply saying what 'colony selection'/'superorganism' says as well - the positive benefit to the queen is what matters.