Should a worshiper, after the Imam, perform the taslim by turning his head towards his shoulders, or to the farthest ends of the row, and is his action valid when he performs the taslim individually by looking towards the ends of his shoulders?

There is no text in the Sunnah (definition) that specifies where the worshiper should look when performing the tasleem (salutation), but it is mentioned in Sahih (definition) Muslim that the worshiper turns to the person next to him. Jabir ibn Samurah narrated: "I prayed with the Messenger of Allah, peace and blessings be upon him, and when we would give the tasleem, we would say with our hands 'Peace be upon you, peace be upon you.' Then the Messenger of Allah, peace and blessings be upon him, looked at us and said: '(What is wrong with you, pointing with your hands as if they are the tails of wild horses? When one of you gives the tasleem, let him turn to his companion and not make a gesture with his hand.)'"

Evidence also indicates a strong turn to the right and left. Amir ibn Sad narrated from his father: "I used to see the Messenger of Allah, peace and blessings be upon him, give the tasleem to his right and to his left until I would see the whiteness of his cheek."

And from Abdullah ibn Masud, may Allah be pleased with him: "The Messenger of Allah, peace and blessings be upon him, used to give the tasleem to his right, 'Peace be upon you and the mercy of Allah,' until the whiteness of his right cheek was seen, and to his left, 'Peace be upon you and the mercy of Allah,' until the whiteness of his left cheek was seen." This only occurs with a strong turn.

Therefore, the worshiper should turn strongly to the right and left, directing his turn towards those on his right and left. He may look at them or at their shoulders.