The only show you may need tickets for is the ice show, they will tell you want day to get them and where, usually a few days into the cruise. Other than that, you will not have to get reservations for any of the shows, just go a little early and get a good seat.
That won't happen. A great number of people don't even decide until the last minute if they will be attending. Other than having ushers to police the isles, this will continue to be an on-going problem. The only way to combat it is to just sit where there is an open seat. If someone is standing there trying to save them, tell them you don't speak English!!!