In this instance, you can replace a player even after the game’s scheduled start time. Be careful, though, because if all other games have started at that point in the day, then there would be no available players to choose from for a replacement.
Depending on the platform and contest, there are situations where you can.
Yes, absolutely, you can play custom matches or custom contests as well as season-long daily lineup leagues.
Yes, DFS is legal almost everywhere in America. Please check with your site of choice to confirm.