Hmm, so someone can fall out of nirvana? If that were so, I don't see why the bodhisattva position would be so important. What would be the great sacrifice of hovering on the threshold of enlightenment if nirvana weren't a permanent change?

Temporary peace and contentment might be possible without enlightenment, but they'd still be subject to dukkha. An enlightened person would be free of dukkha (and of samsara).

Again, what I find arbitrary is that it's only the negative emotions, brought on by the illusion of the ego, that supposedly fall away with enlightenment. I contend that compassion, ambition, progress, and other potentially positive states are brought on by that illusion too, so they should likewise fall away, leaving the enlightened person in a state of nihilism and indifference. You say that's mistaken, as is shown from Buddhist practice, but as is not so easily explained in philosophical terms.