Rukoo and Qaumah

Raising the hands before ruku'

Q: My question regards the raising of the hands before and after ruku. I have read that there is not sufficient evidence to prove that Nabi (Sallallahu Alayhi Wasallam) did not perform this action during Salaah. The same applies to the positing of the hands during Qiyaam (on the chest or below the navel). I would be indebted if Mufti could please provide supporting evidence.

Keeping the back stright in ruku'

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?

Reciting ربنا و لك الحمد حمدا كثيرا طيبا مباركا فيه in fardh Salaah

Q: Can a person read the following Dua in his Fard Salaah:  حمدا كثيرا طيبا مباركا فيه after reading: ربنا و لك الحمد, in ruku', similarly can he read in jalsah: اللهم اغفرلي و ارحمني و عافني و اجبرني و ارزقني and if he is a muqtadi can he read these two dua's?