Various Masaail on Qadha Salaah

Q: What is the ruling of Qadha salah? Is there any authentic ahadith regarding this? Is Qadha for those who missed salah intentionally or repentance is enough. In India we make up our salah in all cases whether it is intentionally or any other way, if we miss we make the salah. I saw a saying if Ibn  Taimaiyyah : Volume 2, Page 99a: In law, there is no way for one who leaves a salah intentionally to make its qada' 

Q: If Salah is not valid, how does one then repeat it thereafter individually or what should be done?

Q: My friends insist that there is qadha for missed Salaah during haidh just like there is qadha for saum. I tried explaining to them that there isn't but all they say is that I am not learned and the learned have told them. Its reached a stage where they are calculating their missed Salaahs and making qadha of it.