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?

Muslim civil engineer constructing buildings for armies fighting against Muslims

Q: I am working as a civil engineer. We deal with construction of buildings and other structures. Sometimes we have to work for the army of disbelievers (kuffar) who are presently fighting against Muslims. We have to construct buildings and other structures for them... Is it allowed for us to work for them although we have no intention to help them on their anti Islamic sentiment. Remember our land is under oppression.