That blend of Realism/Idealism in Advaitic Epistemology/Ontology is only intelligible from a perspective of non-numerical identity. Even in Western philosophy those slashed categories are not watertight compartments but in Advaita the being or nature of reality is the foundation of what the author of Vedanta Paribhasa, Dharmaraja Adhvarindra, calls its perceptuality or capacity to be perceived. For instance the intractable mystery of the unknown object for Berkleyean Immaterialism is avoided by holding that the reality of something as an independent entity is based upon the possibility of its being an ‘unknown object’/ajnanatta satta(bring your own diacretics). Something that comes into existence in the moment of the awareness of it does not have the independent status of the object which waits unknown to us.
What then of what has been called the snake/rope illusion, more properly confusion? Viewed from a Western perspective this seems to be the argument from illusion as a narrative. I speculate that neo-advaitins that have come under the influence of Descartes are led by the nose in that direction. However if you put yourself into the mind of the Advaitic lateral thinker as above in the ajnanatta satta the vertiginous thought that confusion is the perfect analogy for veridical perception may occur. The rope as it is in reality travels into the mind of the perceiver as a snake. It is superimposed on his mind as such for the duration of the confusion. The basis of the reality of that confusion is a real rope out there in the world. The language of ‘seeming snake’ is not retrospectively used to undercut our everyday undeluded experience.
The argument proceeds analogically. As in the case of admitted confusion the mind takes the form of the object or technically a mental modification/vrutti. The default presumption is that the perception is veridical. No one sees a seeming snake or diabused of their confusion a seeming rope. Somehow the object of perception is in the mind of the perceiver. The rock I am looking at is in my mind but not as a material rock. The ontology of the rock or its nature such that it can be a mental modification brings us to the concept of non-numerical identity. For another post.
My compression of the argument here is deliberate. I look on argument as a ladder. How is the correct distance between the rungs arrived at? It is assumed that ascent is twice as demanding as walking along on the ground. The normal stride is about 2 ft. and a bit. therefore the ladder rungs are 1 foot apart. Scholarment likes lots of rungs, the space of argument being infinitely divisible. One loses the sense of the argument as a whole by getting involved too early in discussions of a-khyati (no-knowledge).(cf. Perceiving in Advaita Vedanta by Bina Gupta pub.Motilal Banarsidass. Delhi Chap.6:The Nature Of Error.)
From every point of view, however, there is no difference as regards the appearance of one thing as something else. And in accord with this, we find in common experience that the nacre appears as silver, and a single moon appears as two.(from Preamble to Brahma-Sutra-Bhasya by Shankara translated by Swami Gambhirananda pub. Advaita Ashrama)