What is the ruling on kneeling during prayer when sitting for the Tashahhud or between the two prostrations for one who is unable to bend their knee or finds it difficult, yet is able to sit cross-legged, and what is the ruling on their previous prayers?

The prayer is valid, and there is no specific posture required for sitting between the two prostrations or for the Tashahhud. Rather, sitting in the postures of Tawarruk and Iftirash is recommended, and it is permissible to sit in any manner, due to the generality of the legal evidences.