Your interpretation would be correct if the sentence had said "think" instead of "suppose". However, "suppose" changes the meaning. A supposition is a special kind of thought. A supposition is a thought based on speculation, surmise, or guesswork. My interpretation of your sentence is this: "On the basis of the little he could remember of her, he could only speculate how intelligent she might have been. The few clues available from his memory were not enough to suggest that she was, in fact, unusual or intelligent." Actually, she might have been very intelligent. He doesn't suspect that to be so, but he's not ruling it out either.

Yes you’re correct

Both

Yes, you are correct! The sentence means that based on the speaker's memory of the woman, he does not believe that she was an unusual or intelligent person. The phrase "still less" adds emphasis to the idea that the speaker did not consider her to be intelligent, suggesting that being unusual would be a step up from his perception of her.

assume