To put things more plainly, it’s a routine observation that 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 expressed 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 is only that one audience may require us to articulate what goes without saying, what is understood in another. It may be useful exercise to unfold the implicatures and presuppositions that are taken for granted in another discourse situation, but giving a name to one’s habitual position is not the same thing as a change of address.
So, yes, I would have to say that Peirce was 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.