Sitting on a chair and performing Salaah

Q: Many persons due to medical reasons decide that they need to use a chair for Salah with Jamat and they do not consult a reputable scholar, thinking that a doctor's certification or a medical note is sufficient justification.

1. Is this rationale correct?

2. What is the status of a person's Salah in Shar'iah if he uses a chair for Salah when in fact he is not entitled to?

3. Exactly when is a person permitted to use a chair for Salah in Jamaat or otherwise?