Qiraat in Salaah

Q: I am currently becoming hafiz at the age of almost 40 (please make dua that I reach completion). I practice what I have learnt during the day by reciting it in namaz and in nawafils. After Esha I come home from the masjid and I recite quietly in the sunnats of Esha and loudly in witr namaz and in nawafils thereafter. I do not pray loud in nawafils during the day because I have been told that you can't recite aloud in the sunnats of (say for eg. Esha, maghrib) and that you can't pray loud in nawafil during the day (only at night), is this true?

I find it so much easier and more helpful if I read aloud, I find that reading aloud helps me retain what I have memorised much better. For the sake of my purpose (hifz-e-quran) would it be ok for me to recite aloud in sunnats of maghrib/Esha, etc and in Ishraaq and chast namaz in the privacy of my own home (without disturbing anyone)?

Q: One who reads only the word ha meem or yaseen in salaah, will it suffice for the fardh of qiraat?

Q: If one has to recite only one Ayah in a rakat of a Salah how long it should be?

Q: Can Surah Ikhlass be recited in the first rakaat and either Surah Falaq or Surah Naas in the second rakaat?

Q: What about specifically misprounincing the letter ذ as ز , ظ or the letter ث as س , ص ? For non-arabs the letters ذ , ظ , and ث are quote difficult to pronounce since these letters are prounounced with a lisp i.e bringing the tongue between the upper and lower teeth when pronouncing it. So the non-arabs because they are not used to the lisp sound read the same representative lettersound without the lisp. In such case is the salaah invalid either intentionally or unintentionally?

Q: Letters which have similar sounds like: ذ , ز , ظ does mixing up between them nullify the salaah? Also ت , ط and also ث , س , ص and also د , ض and also ح , ه and also ا , ع also ق, ك does mixing up between these letters nullify the salaah? I read that there a different views, but what is view that is to be taken ? And what did the majority say?

Q:

1. Is it imperative to read diff rent surats each day in the five salaats? Would salaat be valid if one continuously recites the same surats only.  For example, in the Maghrib salaat each day, one habitually recites sura Falaq in the first rakaat and sura Naas in the second rakaat. 
2. Can Ayatul Kursi be recited in place of a sura in salaat or is it necessary to add at least two ayaats before or after ayaatul Kursi?

Q: I have a question regarding Salaah. For eg. In Esha Salaah there are four rakaat's, in which two are audible and two are silent. What should we do in the last two rakaats?