To put things more plainly, it’s a routine observation we have no need for moods and tenses in actually doing mathematics, that is, in developing the consequences of given axioms, constructing formal models, or applying models and theories to the applicable phenomena. Theories of change, intention, and possibility can all be stated in present tense indicative mood. Regarding change, intention, and possibilities as real or not is independent of the linguistic forms we happen to use in their description.
Audiences, interpreters, receivers are neither right nor wrong. It simply happens that one audience may require us to articulate what goes without saying, what is taken for granted, “understood” in another context. It may be useful exercise to unfold the implicatures and presuppositions taken for granted in a particular discourse situation, but giving a name to one’s habitual position is not the same thing as a change of address.
So, yes, I’d say Peirce is a realist about possibilities, and patterns of possibilities, from the start. That much is simply implicit in his mathematical approach to logic, probability, and information.