Should the worshipper stand abreast of the Imam or slightly behind him if the congregational prayer consists of only two individuals, and is it authentically reported that Umar (may Allah be pleased with him) once brought someone forward to stand abreast of him after that person had been standing behind him?

If there is only one follower (ma'mum), he stands to the right of the Imam, slightly behind him according to the majority of scholars. It is also said that he stands abreast with him, and this latter opinion is the more sound one. This is based on the authenticated report from Umar (may Allah be pleased with him) where he brought forward someone who was praying behind him until he was abreast with him.