My daughter is 9 months and is still working on complete head control. She was also born via scheduled C section. She most probably has Cortical visual impairment. She will look at people, but not really focus on them most of the time. She's been rolling over for about a month now, but like your daughter, is not even close to sitting up. We have already been through the battery of tests in Pediatric Opthamology and pediatric neurology (she had an MRI at the beginning of December) She is just starting to make consonant sounds, and will always grab fingers, but rarely will she hold on to something you place in her hand. She does not explore and grab things at all.
I have been in contact with:
1) "Blind Babies" (for babies with ANY type of visual impairment)
2) "The Regional Center"
3) "CCS" (California Children's Services)
These three agencies provide assistance and stimulation and can get you on track with Physical Therapy and Occupational Therapy (yes, PT...AND...OT - trust me, it's important.)
What the neurologist said happened was that as her fetal brain developed, a type of brain matter that is supposed to develop in the center of the brain, then migrate outward, did not migrate to where it was supposed to be.
Basically, there are "two" types of brain matter. White and Gray. They both have specific sections they are supposed to occupy in the brain. My daughter has some Gray Matter where White Matter is supposed to be. This affects her Motor and Visual Cortexes.
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EDIT:
DO NOT let doctors / well-meaning family and/or friends try to advise you against agressive, early intervention. The earlier you help your daughter, the better off she will be.
I knew my daughter needed help by two months old. Her doctor said wait at least 6 months before trying to see if she was delayed. We didn't want to lose four months where she could possibly be getting help though, so we took her to Children's Hospital, Oakland. The doctors at assured us we did the right thing in trusting ourselves, because no one knows your child better than you do.