Where should the chair be placed for the worshipper in the row, and does the chair being moved backward affect the prayer of the row behind it?

If a person is excused for not standing and prays seated on a chair, their excuse does not permit them to omit bowing (ruku') and prostration (sujud) if they are capable of performing them, and vice versa. The general rule is that whatever the worshipper is able to do, they must do, and what they are unable to do is waived.

As for the placement of the chair in the row: - If one prays seated throughout the entire prayer, the consideration is given to the position of their sitting (their posterior) to align with the row, such that the back legs of the chair are aligned with the legs of the worshippers. - If one prays standing and sits down for bowing and prostration, the consideration is given to the standing posture. They should align with the row while standing, and the chair should be behind the row in a position that does not inconvenience other worshippers.