Skipping a surah in salaah

Q:

1. Can we skip surahs in between in salah? If not, for which salah (fardh or sunnat) can we skip surahs. Example, can I read Surah “Al-Kafirun” then skip Surah “An-Nasr” then recite Surah “Al-Masad” in first and second rakat.

2. How many surah's can we skip in between in Salah?

Backbiting

Q: Is it kufr to backbite a Muslim to a non Muslim? Also is it kufr to complaint against a Muslim to a non Muslim if he has harmed you for the purpose of correcting him so that he doesn’t do the same mistake again?

Questions relating to a masbooq

Q: I have a few questions regarding a late comer joining salaah:

1.Is it important to recite sana once you standup to complete your prayer?

2. What if sana was recited when the person joined in the 2nd, 3rd or 4th rakaat

3. Due to misunderstanding and following someone, if a person follows a different procedure to complete the missed rakaats in salaah for a couple of years and now he comes to know correct Hanafi procedure, will his passed salaahs be valid?

4. Is the below procedure correct as per Hanafi mazhab: If a person joins jamaat salaah and says salaam at the end of the 2nd rakaat and after salam he stands up and doesnt read sana and treats this rakat as the 4th rakaat. He only recites surah fatiha and completes his salah. Is his salaah valid or does he have to stand up recite sana, surah fatiha and a surah to complete his salah?

Adoption

Q: We are Muslims and a childless couple. We adopted a baby girl and we don't know about her parentage. We want to give her our name and identity and raise her as our own, without telling her that she was adopted. We don't know who are her parents. Is this allowed?