What is the posture for sitting between the two prostrations and during the Tashahhud? Is it forbidden to flatten the feet and sit on them? And is that considered "Iq'aa'" (squatting)?

It is sunnah to sit between the two prostrations in the Iftirash position, which is by folding the left leg and sitting on it. And spreading both legs between the two prostrations is not forbidden, but it is contrary to the Sunnah, and it is not considered Iq'aa. Iq'aa is to set both feet upright and place the buttocks on the heels, and this is recommended between the two prostrations, but the best is the Iftirash position.