Is it permissible for a worshiper to sit between the two prostrations and during the first and last Tashahhud on the tips of his toes, pointing towards the Qiblah, without Iftirash or Tawarruk, and does that affect the validity of the prayer?

The session of al-iqa' (squatting) which involves planting the feet and sitting on the heels between the two prostrations is permissible or sunnah (definition), and is not forbidden. There are two types of al-iqa': the first is disliked (makrooh), which is when the worshiper sits on his buttocks, raises his shins, and places his hands on the ground; and the other is permissible or sunnah, which is when the worshiper places his buttocks on his heels, sitting on them and on the tips of his toes. As for the tashahhud (definition), this sitting posture is not sunnah, but if the worshiper sits in this manner, his prayer is valid.