Is it permissible for me to return to the first row on Friday [for prayer] after having left it due to an emergency, even if that entails stepping over rows?

If you leave your place for a need, such as performing ablution, you have the right to return to it and you are most entitled to it. In this case, stepping over people's necks is not disliked, for your ruling is like that of one who sees an opening in front of him, due to the Prophet's (peace be upon him) saying: "Whoever stands up from his place and then returns to it, he is most entitled to it."