Is it obligatory for me to alert someone who did not return the greeting of peace (salaam) or returned it with a wording other than the prescribed one, to the obligation of returning the greeting of peace with the wording "wa 'alaykum as-salaam," and would I be sinful if I did not do so?

If you greet a Muslim and he does not return the greeting, advise him gently and explain to him the obligation of returning the greeting, for advice is one of the foundations of the religion. Advice is an individual obligation (fard ayn) according to some scholars, and a communal obligation (fard kifayah) according to others. It becomes obligatory to the extent needed if the advice is accepted and the adviser is safe. And whoever does not perform it, despite having the ability to do so and no fear, is sinful.