This sequence is progressing well!

You correctly pointed out that we don't yet have very strong evidence that our inner senses supervene on our external senses. While neuroscience is maturing, all we have is evidence of at least a partial relationship between the inner senses and the external senses, and a nonzero prior belief that supervenience may in fact obtain.

I think we have reason to have a very strong prior belief that supervenience obtains. Suppose we had an extra external sense that had, so far, evaded scientific explanation. Imagine being able to sense the presence of dark matter, or psychic auras or something. Based on our previous successes in analyzing sight, hearing, and so on, wouldn't we be right to feel confident that this extra sense did in fact supervene on the other senses, and that we would eventually figure it out?