Its of course conceivable... but i doubt there are people who can be described as such.
If you were illiterate you could still be taught mathematics and the symbols involved, i.e. learn verbally, or experimentally etc... but being taught math, or learning it yourself would imply a situation or disposition that would lend itself to learning written language first, and more importantly.
Probably not. As far as I know, most reading disorders, such as dyslexia, are quite a bit different from functional illiteracy. Correct me if I'm wrong, but illiteracy is the result of a lack of education, not a learning disability. And if you don't pay attention in school, you're not going to learn any math.
I don't see why not. Some people just seem more "wired" for math than others. It's kind of a pointless question though, because math isn't something that's carried out in seclusion in people's minds and then opens a portal to Plato's world of forms. It's a social activity, and you won't learn math or be able to communicate the math you invent without literacy.