If prostration involves slight difficulty, and sitting between the two prostrations causes very severe pain, is it sufficient to sit on a chair and gesticulate after bowing, or is it obligatory to prostrate, then sit on the chair between the two prostrations, then prostrate on the ground, taking into consideration the hardship that this may cause?

Prostration (sujud (definition)) is a pillar of prayer and is not waived except in the case of incapacity. Inability to sit on the ground does not waive prostration. So, whoever is able to prostrate should prostrate. Then, if he is unable to sit on the ground, he should sit on a chair, because "the waiving of one obligatory act does not waive another."