I’m wondering whether there is a distinction to be made between “partial communion” and “partial membership?”
Possibly, if you believe that a certain communion could be attained without membership in the Church, but what's certain is that membership is more restrictive than communion, and anyone who believes that infidels, heretics, and schismatics can be saved, must also then believe that infidels, heretics, and schismatics are within the Church (since there can be no salvation outside the Church). So that means they can be in communion with the Church, and, since they're not actual Catholics, the communion can only be a partial one.