How can one distinguish between the outer and inner parts of the ear canal with a clear and practical criterion, and is earwax considered a barrier that must be removed during ghusl, despite being natural and a bodily secretion?

It is not obligatory to wipe what is hidden of the ear during ablution, for what is apparent is what is normally seen of the ear, as opposed to what is concealed within it. Straining to get water to what is hidden of it is not required. Some jurists have explicitly stated this because, in the case of the original head, it is not obligatory to wipe what is concealed by hair, and the ear is even more so. Moreover, water reaching the inner ear might invalidate the fast of a fasting person. Exaggeration in washing the ear canal causes hardship; it suffices to wash what is visible of it without delving deep. Some jurists believe that washing the ear canal is not obligatory at all. The strong presumption that water has reached is sufficient, without undue effort. Removing hardship is a legitimate objective of Sharia (definition), and slight dirt or a minor barrier that does not prevent water from reaching, like dye on a fingernail, does not harm. Removing visible earwax without probing, and passing water over the outer ear, is sufficient.