Is failing to alert a worshiper who prostrates incorrectly a sin that necessitates bearing the burden of his prayer, and does verifying his prostration during prayer invalidate it?

Placing the elbows on the ground during prostration (sujud) is disliked (makruh), but it does not invalidate the prayer. One should advise the person who does this and teach them the correct method if you are certain of it. There is no blame upon you for not alerting them. Looking to verify the posture of prostration is not considered exiting the prayer, but it goes against proper devotion (khushu').