Refraining from areas of doubt

Q: I loved a girl. We both were in love for sometime and family come to know about us. My family accepted our relationship and my mom asked there daughter for the marriage for which they refused. We are not in touch after that. After one year they sent us the proposal that they want to marry this girl to me. But I come to know some strange facts that she also had a relationship with her uncles and she also has boy friends. I didn't know she was a cheat. I have all the proof that she used to talk with her cousin brother and uncles over the phone. Please advice me. Is it permissible to marry this kind of girl? 

Becoming certain that one did not err in Salaah after initially having a doubt

Q: If a person doubts in his prayer, he should act upon that which he is certain of, or upon what is weightier in his mind, and do Sajdah Sahw at the end. But what if it becomes clear to him (lateron during the prayer) that the course of action he has proceeded upon is in accordance with reality and that he has neither added to or deleted anything from his prayer. Should he still do Sajdah Sahw or not anymore?

I read that Shaykh Uythamin said: "... he no longer has to perform the prostration for forgetfulness, due to the fact that doubt which necessitates it is no longer present.

A different saying is that it is still necessary to perform the prostration of forgetfulness in order to humiliate the Shaytan, due to the saying of the Prophet (peace be upon him), "And if he has prayed it completely then the two prostrations will be a humiliation for the Shaytan." 

And due to the fact that he performed a part of his prayer in a state of doubt with regards to its performance and this is the more correct saying insha'Allah.

An example of this is if a person prays and doubts in a rak'ah, whether it is the second or third rak'ah. Neither of these two possibilities carries more weight in his mind, so he makes it the second rak'ah and completes his prayer upon that basis. But whilst continuing, it becomes clear to him, that it was the second rak'ah in reality. In such a case there is no prostration for for forgetfulness due upon him in the saying most well known in the madhhab, but prostration for forgetfulness before the salutation is required from him and the second saying that we hold to be preferable."

What if such cases do happen often? Like when one does stand up for the 3th rakaat one does doubt and does think he missed the first tashahhud then in the 4th rakaat he remembrs he did the first tashahhud. 

Communicating with the boy before nikaah

Q: I met a boy and spoke to him for the purpose of marriage. It didn't take me long after that to decide that I would like to marry him. My parents did not have a problem and were happy as well. I have now been speaking to this boy for over a year because he says he still needs time to decide. According to him, he does like me but he is just very confused about getting married. Please advise me on what to do as I know now that it is haraam for me to continue speaking to him. I did try stopping but he always says that he will decide soon and he also gets upset and has a problem eating and sleeping when I do stop. Please advise on what he can do as well to help him decide.

Inheritance

Q: My brother and my mother are using my father's property as they will and not letting me to use it. Always making some kind of reasons. What shariah suggests me to do?