Well, no, I'm not saying that just plain water soaking would help. Why do you think cowashing daily would be bad on your hair exactly? If you have dry coarse hair, it needs more moisture. To do that, you're going to need to condition it a lot more often than its currently getting. I have low porosity hair that is resistant to deep conditioning too, but I've found that the daily lighter conditioning helps it, as does steam deep conditioning. I don't have a steamer, but I wrap a hot, wet towel around it for about thirty minutes.
Basically the bottom line is with your hair being dry and unmanageable, it needs more moisture and it just sounds like your current routine isn't giving it quite the moisture it needs. Hair loses moisture, so a daily cowash (which is just a daily condition basically) would help dramatically.
As for deep treatments, with dry hair, have you tried any oils? An overnight olive oil, coconut oil, or avocado oil treatment might be just what your hair wants, rather than protein.