Actually, I really still don't get this.
"She'll misunderstand if not already" means "she'll misunderstand if (she has) not already (misunderstood)." But the text says:
"если уже не поняла" which means "if (she) already didn't understand", which is different from "if she didn't already misunderstand."
So, a literal translation would seem to be: She'll misunderstand, if she already didn't understand. Or, more likely: She'll misunderstand, if she didn't already understand.