Hi Bohiruci,
I am not familiar with the Culavedalla Sutta so I will just share what is in my mind after reading it.

First, I think it is unlikely that Visakha, who has attained the stage of Anagami, is confused with what Buddha meant by the term "self". I will take it that Visakha is seeking for the deepest level of understanding on the doctrine of ‘no-self’ from sravaka viewpoint -- progressing from a third stage of holiness to the final stage of Arahatship that Dhammadinna has attained.
If we were to ask what is ‘self’? A series of questions may follow like the case of Visakha. This form of enquiries will continue without end like a never ending samsaric cycle. It is analogous to rebirth where the previous question serves as the cause and the answer serves as condition for the next question to arise. We witness old mental tendencies in action, replacing one fragment of thought with another fragment where every fragment is unsatisfactory during the process of enquiry. How is it that we can't even know who we really are? What exactly is lacking? If we were to replace the question to “without using any languages, any symbols to represent ‘I’, how is ‘I’ experienced?” To answer such a question, the old chain-cycle habit of answering must come to a halt. To answer, the mind cannot continue its chats. The mind must be silent, raw, undefined, direct and attentive – it must be mindful. A question arises, the answer isn’t made up of words but rather a raw direct naked experience of what it is, i.e., using mindfulness as the way of knowing.
The entire teaching of Buddha is very much alive. When Buddha taught us ‘impermanence’ and ‘interdependence’, he does not only teach us the analytical and logical mode of knowing the seals but also the direct mode of knowing -- being participative and experiential in all moments. He taught us the four foundation of mindfulness that brings about clear knowing that eventually leads to nirvana. It is amazing to realise the immense clarity that arises in mindfulness, knowing that arises not out measurement and comparision. Apply mindfulness as the mode of knowing in all that can be experienced – body, feeling, perception, mental formation and consciousness. Leave nothing out. It is through this sort of concentrated mindfulness of phenomena that can give rise to the flow of clear knowingness that comes only with the absence of words, labels and perceptions. This signless clear knowingness is release, it is bliss, it is what that leads to the other side of unbinding:
"What lies on the other side of ignorance?"
"Clear knowing lies on the other side of ignorance."
"What lies on the other side of clear knowing?"
"Release lies on the other side of clear knowing."
"What lies on the other side of release?"
"Unbinding lies on the other side of release."
"What lies on the other side of Unbinding?"
-- Cula-vedalla Sutta