"Ahh, yes, Israel ATC this is UA90, we've got 2 passengers that have relocated themselves from the back of the plane to the front, and are refusing to identify themselves or show their boarding passes or any other form of identification"

Whilst flying into an airport where there is a legal requirement that all passengers (and crew?) need to be seated and remain seated for the final 30 minutes of the flight, and several other security-related requirements. Whether they would have taken any action after the event, who knows - but I'd be fairly sure they would have taken action at the time!

One advantage to flying El Al I guess - the armed air marshals on board would have likely been more successful in asking them passengers to return to economy than the UA staff would have been.

I've seen a few. The first similar to your second one where an elderly lady was sitting in my seat in row 1. I suspect this was simply someone not aware that the aircraft was assigned seating and had simply taken the nearest seat to the front - or at least, that's how she acted when asked to move. On 3 occasions I can recall I've seen people move to seats in international C after takeoff (from memory, all after the meal service when most of the cabin was asleep), and in all cases were moved back to Y fairly quickly once the crew noticed them. In one of those cases they had moved to a pilot rest seat, which ended about as well as you'd expect...