Is it permissible for someone who has reserved a place in the first row after Maghrib prayer to leave and then return to it, and is his action considered stepping over people's necks?

If someone leaves his place in the mosque for a need and then returns to it, he has a greater right to it, and the one who sat in his place must stand up for him, unless he left it without an excuse, in which case his right is forfeited.

As for stepping over people's necks to return to one's place, the correct view is that it is forbidden except for an excuse. Among the excuses that permit stepping over necks to return to one's place is doing so quickly and with the permission of the people.