I am not sure whether Louise was 4 or 6 weeks early, but prematurity does have a link to the development of esotropia and exotropia, but I think Louise may have had the condition had she been born at term. Congenital strabismus (esotropia or exotropia) does not occur at an increased rate in premature babies. The condition is caused when the muscle in the eye(s) are weakened or there is a defect with it. Her sudden birth due to a placental abruption MAY have caused the condition, due to the lack of oxygen to the fetus when a placental abruption occurs, but it is quite unlikely.
The articles all state that Louise has exotropia, however that conditions is when the eyes turn outwards; Louise's eyes turn inwards which makes me think she has esotropia, not that it matters really.
I don't think Edward and Sophie will choose to get her condition fixed, as it is known to be more effective when done under the age of 5. I think they are incredibly protective of Louise, Sophie in particular. Parents worry about their child going under a general anaesthetic, no matter how routine it is, and given the circumstances of Louise's birth, they are probably both quite reluctant. Sophie will have experienced agonizing pains with a placental abruption, and quite probably bleeding; therefore she probably thought she would lose her child so to find she had a healthy baby girl probably makes her all the more careful of putting her through anything that could risk losing her. I have seen placental abruptions occur when the unborn baby has not survived (I am a midwife) even at full term, so I can understand why they are protective of their daughter.