ANSWER: The father in law is the permanent mahram.  Even if his son divorces the woman or if the son dies then also the father in law is a permanent mahram for life for this woman.

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ANSWER: No, you must not make up that rakah.  You are exempted from reciting fateha and this rakah is valid if you caught the imam in ruku and you got to say the prescribed dhikr at least once. 

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ANSWER: You can recite it silently, providing you at least move your tongue and lips.

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ANSWER: No, you must not recite tashahhud after making sujood as sahu. You directly make salam.  https://www.assimalhakeem.net/sujood-as-sahu-prostrations-of-forgetfulness/

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