You have to stay within the red circle for two minutes to get the raid to time out. If you leave the circle, the timer will pause until you get back. If you run in and out of the circle, the mobs will hang around smashing your stuff and the raid will seem to take forever. If no one is around to load that area then the mobs will "not exist" until someone loads that zone again. I don't play multiplayer so I don't know all the finer details of what happens when your friend walks by your raid.

Dig a ditch with steep edges around your base and make sure the inside is covered by the workbench area of effect. Then you'll only have to worry about flying mobs.

Raids get stronger as bosses are killed in a world. The number of days makes no difference.