Regarding "Line Of Sight", spells are a somewhat less certain situation than bullets. If you rule that he has to stick his head out, then that's that. I have nothing to add to it.
As for players falling off the wagon: I don't really want to blunt it as much as this, but yes. The blame can be laid on no one else's feet. However, life happens. We can move on, and you can recruit more players. Or we can fold up and go home (i.e. start a new game). The call on this is really yours.