Is it necessary to repeat the prayer if the Qibla deviates by about 30 degrees without an excuse? And is it permissible to limit the change in the prayer's posture to the place of excuse only?

A slight deviation does not affect the validity of the prayer. However, the mistake lies in sitting on a chair for the tashahhud while being able to sit on the ground. What the worshipper is able to do, he must do, and what he is unable to do is waived. If the difficulty in prostrating is evident, then nodding (performing īmāʾ) is permissible. However, it is not permissible to sit on a chair for the tashahhud while being able to sit normally on the ground. If she did that, it appears that she violated the posture of sitting, and she must repeat the prayers, because she did not perform īmāʾ from the normal sitting position, which she is capable of. Some scholars are of the opinion that the prayer is invalidated if one does not perform what he is capable of from īmāʾ. If the worshipper is able to descend and sit in the iftirāsh or tarabbuʿ posture, he is obligated to do so and not sit on a chair.