Should I join the row with the Imam exposed, or stand behind the Imam and shield him while leaving a space between me and the row?

You must join the incomplete row you found, whether it covers the Imam or not. Do not stand alone behind the Imam with a space between you and the row, because it is from the Sunnah to complete the rows, the first then the next. And because if you pray alone, your prayer might not be valid according to a group of scholars. It is also recommended for the Imam to stand in the middle of the row.