Assalâmu’alaikum Warahmatullâhi Wabarakâtuh
1) It is understood from the text of the Fuqaha that the “Fardh Ruku” which is minimally required in Salaah composes of bending the back to the extent that a person’s hands reach his knees. If this is acquired, then the Salaah will be permissible. Hence it can be understood that to keep the back and legs straight is not wajib in Salaah and Sajdah sahw will not be necessary if one leaves it out.
However, the Sunnah method of performing ruku comprises of bending the back till it is straight and aliening it, the posteriors and the head, keeping the legs straight and grasping on to the knees without bending the arms. (Umdatul Fiqh vol 1 pg. 422 Halabi Kabeer pg 244, Durrul Mukhtaar vol 1 pg 493-494)
2) To keep a space between the legs and the stomach in Sajdah is also among the Sunnah acts of Sajdah. It is mentioned in the Hadith of Muslim that in Sajdah Rasulullah Sallallahu Alaihi wa Sallam would keep a gap between his stomach and his legs to the extent that if a lamb wanted to pass through, it would be able to pass through. However, in the course of Sajdah if one doesn’t do that, Sajdah Sahw will not be required.(Halabi Kabeer pg 280, Umdatul Fiqh vol 1 pg. 422)
ALLAH TA’AALA KNOWS BEST