Qiraat in Salaah

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?