The justification is that if you forfeit your floor space at E3 someone else can come in and grab it the next year.
If they keep the floor space this year and eat the cost they still get first dibs on that large area for the future. If they don't someone can swoop in, take it, and Nintendo will have to pick from a bunch of much smaller booths.
I mean, it's pretty obvious Nintendo's retaining their floor space because of booking requirements and not because they really thought Zelda needed all of it, but as long as their confidence in it is justified they shouldn't have too much trouble using that space with just Zelda.
These convention spaces tend to be pretty disgustingly crowded, with lines often getting snaked around in stupid ways and even getting capped/cut because of fire codes. No one's going to complain about having a little room to breathe while they're playing.
Don't forget, Nintendo is suggesting that the media set aside 90 minutes for the two demos. If each person is going to be taking up a station for 90 minutes, then they need a whole ton of stations to be able to let as many people as possible do the demo. That's the reason for all that space - normally Nintendo limits play at one station to 15 minutes at the most to keep people moving.
Well, technically you can buy a ticket for like $900, though they keep that quiet. And I think they've already stopped selling badges anyways. But it wouldn't be a waste to head out there on your lunch break, there's always stuff going on in the parking lot around the center (that's free for everyone), and you can walk in the front doors to check things out inside (you don't need a badge until you go through a second set of doors in the back).
I'm guessing not this year. I remember Todd Howard mentioning in an interview how well the Fallout 4 reveal turned out and how they'll likely keep doing a similar short span between reveal and release in the future - TES VI coming out in 2017 already seems very unlikely.