I grabbed her during "Awakening". She is following me currently, constantly talking about wanting me to lead her home (which is the next quest). That's fine, but I notice she REFUSES to carry items for me (mule). How deep into the Dawnguard questline do I have to go, in order for her to be a "normal" follower/companion, where she will trade items with me, and all? Thank you all so much in advance, and yes, I did search the UESP and wiki pages.

Right now, in the game, you′re just a guy who happened to wake her up when you weren′t supposed to, and wants to get her elder scroll, presumably. Just saying that because I enjoyed her reaction when I tried it, in the second playthrough because I′m so nice that I didn′t even think about doing it the first time (I might have been afraid she would go hostile and kill me with her elder scroll powers).

Anyway,

When you say "she'll join you soon after either way." Does that mean as a permanent, "normal" follower, who will now trade items with me? I don't have to finish the entire questline?

Better than a normal follower because she′s one of the strongest beings walking Tamriel and can′t be killed. ^^ But yeah, way before the end of the questline, one quest after she leaves you. You′ll get tired of her complaining before you can even rid yourself of her. She did have a particularly harsh childhood, so some of us manage to look past it and see the beautiful, beautiful forsworn armor you can equip on her.