What is the evidence that the dirt from the navel, resulting from congealed sweat or bodily substances, does not constitute a barrier for ghusl, and does this apply to the dirt formed in the corners of the eyes?

What you have read about excusing frozen sweat and that it is not considered a barrier is the view of some jurists, due to its frequent occurrence and the difficulty of removing it, like frozen sweat. As for frozen dirt in the corners of the eye, it must be removed if it prevents water from reaching what is beneath it.