If a supposed non-actual infinite mode's existence is necessary, then by necessity-entails-actuality it is actual, contradicting the assumption that it is non-actual.

Once again, even this strong modal conclusion is not equivalent to full-blown necessitarianism regarding infinite modes. Even if every existing infinite mode necessarily exists, could there have been other infinite modes as well? I am not aware of any texts in which Spinoza explicitly rules this out, but it is easy to imagine what he would say. Non-actual infinite modes do not exist necessarily, or else they would actually exist, on the plausible assumption that necessity entails actuality (□p →