Conditions for the validity of Nikaah

Q:

1. Person A tells person B, “I will appoint a wakeel on behalf of you who will marry you to me”. Person A agrees. Later the wakeel marries person A with person B in presence of two witnesses. Is the marriage valid?

2. Amatullah comes to know that Zaid married her with Abdullah in presence of two witnesses. But she did not appoint Zaid as her wakeel. Hearing this, Amatullah did not say a word but accepted the marriage in her heart and started treating Abdullah as her husband. Is the marriage valid between them?

Q: Islam tells us to marry when a boy or girl becomes mature, age 12-13. But according to science, if a girl marries at 12-13 and becomes pregnant then she has to face problems like losing the new born baby, decrease in her growth of height, sugar problems, high BP, etc .

So in these cases, can we delay the marriage till the girl or boy is 19 years?

In previous days, in the 18th century and before that, did it also happen that they used to marry young and these problems had taken place (decrease in height, high BP, high sugar, risk of death, etc.).

Q: Zaid conducted the marriage of Zainab to Shuaib. But Zainab did not appoint Zaid as wakeel. However Zainab heard the ijab and qubool through the phone and then later when she was asked whether she accepted the nikaah with Shuaib, she said that she accepted. However, she did not notice what was the amount of mahr which was mentioned. So she has no idea about how much mahr she is going to get. Is the marriage valid?

Q: I got married a year ago. Nikaah and everything took place but I never sign the certificate or recieve a certificate saying that I'm married. Am I married and how do I go about for faskh if I don't have a certificate?

Q: A person asked his parents to let him get married but his parents refused him even though that person was earning good and has enough money to afford all the expenses. That person was in fear of committing adultery. He asked his parents many times and even one day he told his parents the reason why he wanted to get married. His main reason of getting married was to stay away from haraam but his parents ignored him. He was very upset as his parents refused him. He contacted a few women in order to commit a sin but he immediately realized that he was about to make a huge mistake in his life and he repented and then refrained from committing such an act. He didn't committed zina. He stood steadfast and kept remembering Allah Almighty. He wanted to hide his sins and he wanted to confess only in front of Allah Almighty. What to do if the authority asks him as he didn't committed adultery physically? He's ashamed of talking to such women by text messages and he realized that it is a sin. He has already repented from it. What are the rulings of Islam as there was a talk of committing a sin but refrained and didn't committed it physically?

Q: If a man and woman commit zina before getting married. After their marriage, they had children and realized that what they have done before marriage was a sin. But they do not repent when being married because they dont have knowlegde that before marriage they had to repent.

After 6 years they got to know that they should have repented before nikaah, otherwise their nikaah is invalid. Is their nikaah valid or not as they repented after 6 years of marriage.

Q: If a girl and boy considers each other as husband and wife without proposal and acceptance, will their nikaah be valid or not? I saw in one tv program, there was female advocate who said that it was the opinion of Imam Abu Hanifa that if a man points to a woman in a gathering in front of many people and says that she is my wife and she remains silent, it means that their nikaah is valid. So what is the correct opinion? Will the nikaah be valid or not if any one does like this? 

Q: My mom said that during marriage, the husbands family asks the girl if she had completed reading the Quran in front of an aalim/muallima/ustaadh.

I find no logic behind this, please tell the ruling about this according to the sunnah.

Q: Can a nikaah take place by responding yes to the words like "marry me" and "will you marry me"? What is the ruling of Islam in this matter?

Q: Can a woman perform nikaah of two people like nikaah molvi and nikaah qazi? This type of nikaah will be valid or not if performed by a woman qazi?