Method of Salaah

Q: When one recites Surah Naas in the first rakaat, what should he recite in the second rakaat?

Q: When the hands are tied in salaah, should the fingers of the right hand be kept closed, or slightly apart?

Q: After the shahadah, should the finger be lowered to rest on the thigh or lowered to be above the thigh?

Q: My local imam makes some mistakes sometimes in salaah and I need to know whether these mistakes invalidate the prayer.

Mistake 1) The imam sometimes says hamza/alif Instead of saying ein for reciting quran, does this invalidate the prayer?

Mistake 2) For the name of Allah, Al-Quddus in the quran, the imam changes the seen into a saad-does this invalidate the prayer?

Q: Sometimes in rukoo' I do the thikr and then I straighten myself a bit and hear a crack, so does that mean my back only became straight when it cracked? If so do I do the thikr again? Must the back in sujood rukoo sitting and standing always be straight or at ease straight? Is the neck or head considered in this?

Q: What is the ruling on reciting inaudibly, is there a consensus on its permissibility? Like without noise and just moving the tongue.

Q: Why are zohr and asr called sirri salaahs and the Imam would not recite loudly? Any significance? Please explain.

Q: If a person prays a fardh or waajib  prayer sitting down, without a valid excuse, is his prayer considered invalid? I recently heard from a Mufti that the prayer itself would be valid, since he did not miss a fardh element of prayer by sitting down (and not standing). What is the position of the Hanafi madhab in this regard?

Q: How long does the Aa in Aameen have to be stretched for?