Meaning of the Hadith that the Jews and Christians will be burdened with the sins of the Muslims

Q: In the Qur'an, Allah said that no one will be burdened with the sins of another on the day of judgement but a hadith in Sahih Muslim said that the Jews and Christains will burdened with the sins of the Muslims on the day of judgement.

Abu Burdah (Radhiyallahu Anhu) reported the Messenger of Allah (Sallallahu Alaihi Wasallam) as saying: There would come people amongst the Muslims on the day of resurrection with sins heavy as a mountain and Allah would forgive them and he would place in their stead the Jews and Christains. Sahih muslim 2767d Sahih muslim, vol 6 , book of repentance, hadith 6668. 

Please clear this contradiction.

Woman cutting uneven hair to make it the same length

Q: A lady has long hair that she does not cut because it is impermissible. However, over time, her hair has become uneven.

1. Is it permissible to cut the hair so that it looks even (of the same length) from her back when she unties it and puts her hair to her back?
Note that since individual hair strands grow from follicles at different parts of the scalp, making sure the hair looks the same length does not mean that each strand is of the same length. She feels like it is impossible to guarantee that all hair is the same length.

2. She has a lot of split ends and her hair is very dry and unhealthy at the ends. Is it permissible for her to cut the dry part (bottom 3-4 inches) so all of her hair becomes healthy?

Islamic ruling on niqaab

Q: I reverted to Islam 5 years ago. I have a doubt regarding the clothing of women. What is the actual clothing of a woman according to Quran and hadith. Some say that niqab is compulsory and some says it is not. I know it's better to wear niqab. But I want to know what actually Allah says. What is compulsory or what is the sunnah.