Aborting a baby due to deformation of the baby

Q: My sister was 6 months pregnant when a scan showed the baby was not growing well. The doctor said this is a case that the baby will not grow properly. The baby has no lungs and is just alive on the mothers breath. The doctor said if you wait for normal delivery there will be some risks in the delivery and also to the mother's health. The doctor advised to terminate the baby. And we accepted the advice. The baby was delivered and in a few hours died. Sometimes I feel it was not the right thing we done. Maybe if she delivered on her own time then the baby would be alive. 

What is the Islamic ruling? Was what we done a serious sin? If yes, is there a chance of Allah's forgiveness?

Wife taking her husband's surname

Q: I have heard that you are not allowed to take your husbands surname after getting married according to these verses: 

"Call them by their fathers names; that is more just in the sight of Allah. (Quraan 33:5)" 

The Prophet (sallallahu alaihi wasallam) said: "Whoever claims to belong to someone other than his father has committed an act of disbelief. (Sahih Bukhari 3508, Muslim 61)". 

a. Are these quotes correct? 

b. Is the statement you're not allowed to take your husbands surname after marriage correct? 

c. I know of people who have registered themselves for marriage at Islamic centres and they have asked women if they want to take their husbands surname.