1) it is better to read it as early as possible but it is not necessary to do so.
this means that, for example, if u miss ur fajr prayer, unintentionally, then it is permissible to catch up with it when u do ur Zuhr Salâh, but it is better for u to do it before that, as soon as u can.
does that make sense?
2) If one prays the salâh of that specific time before catching up the qadâ' of the previous salâh, then it is invalid, except if he forgets.
this means for example, if u miss ur fajr and u need to catch up with that before u do ur Zuhr Salâh, if u do the Zuhr Salâh before u catch up with the fajr u missed, then the Zuhr u do is invalid. however, if u forget that u had to catch up with fajr and u pray ur Zuhr Salâh, then it is ok.
brother, this is according to the Hanafi madhhab, i am not sure if it differes in any other madhhabs.
i shal post the next bit after this part is understood, 'in shâ'a-llâh :-)