1) What special qualities of these new lamps make them able to withstand the destruction that came to the others? Can they be considered "better" in a way?

Maybe being piloted though the sky by Maiar helps.:)

2) It seems that each creation of a light source is somewhat inferior
than its predecessors. Is this merely an expression of nostalgia and
regret of later generations looking back to a perceived "golden era"?
Or does it represent a true downward trend in creation -- moving from
more perfect to a flawed form?  Is this how elves see history? Is this
how Tolkien sees history?

Noo, and yes. I personally cannot see the lamps of Trees as better than the Sun and Moon. I think nostalgia does give the Elves and Valar a less objective point of view. As for downward spiral...no. I think life is more a convex curve. Born, mature, die sort of thing. I wouldn't call maturing a downward spiral. Civilizations are the same. And generally, we think of modern inventions as "better". I.E. the internet or telephone is better than the telegraph. It may be that the Valar saw the Trees as their "peak" or Golden Age in the curve; they've produced their magnum opus in THAT arena, so to speak. As to how Tolkien saw history, I don't really know. I get the impression he was rather depressed about industrialization, but I don't know whether he thought the Roman Empire era was decidedly better than the British Empire era.

3) What of the notion of progress and improvement from one generation
to the next?  If the view of a slow downward spiral is accurate, what
can be expected for the future? What of hope?

The answer is pretty obvious, and the only reasonable Hope possible in that case is if you believe in a loving God and a Heaven. Otherwise, to hell with Hope!

"I cannot come!" they heard her cry."I was born Earth's daughter." ~The Last Ship

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