Method of Salaah

Q: After the Fardh Salah the Imam makes Du'a in which many if not the majority of people participate, arguably making it an Ijtimai Du'a. Thereafter, the Imam stands up and gives naseehat to the assembled congregation, thereby delaying the Sunnah prayer. From the end of Salah to the end of the naseehat is between 4-5min. It may be contended that nobody is obligated to sit for the naseehat. However when the majority, if not all the people sit for the nassehat, it makes it
awkward for one or a few individuals to depart to perform their Sunnah as:

(a) People are in the way and get inconvenienced.

(b) Persons draw uncomfortable attention to themselves by getting up and leaving.

Is delaying the Sunnah for this duration or at all permissible?

If not, what is the proof that such a delay is impermissible?

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?