In answering your first and main question I feel rather torn. Whilst I would really love to see them one more time - especially now I know they can actually still cut it after all these years, I really want them to end on a high; The O2 show. - Rather than fizzle out half way through a punishing tour, where failing voice, exhaustion or whatever have hampered the schedule, with the media rubbing their noses in it.
On the other hand they do what they do. And what they do is be Led Zeppelin and they're only doing that because they enjoy it. And who are we to tell them otherwise?
I'm sure they'll only agree to go and record something if they've actually found they've created something worth recording. They're well aware that an entire back catalogue and heritage could be compromised by an ill-advised missing of expectation.