Nenya or the Elessar?

[Gorthaur the Cruel]

That seems to obliquely speak of the ultimate futility of Galadriel's works in Lórien

How so? Treebeard regarded the Golden Wood fair. They're behind because the workld outside was decaying and growing less beautiful.

The power of preservation the Three provided was a temporary shield against decay, and could provide a simulacrum of the Undying Lands. In the end though, the effort to arrest change in a world in which change was the natural state of things only led to sadness, as the closer they got to Valinor, the more aware they became of the differences between what they'd done with the Three, and the 'real thing' in the West.

I never got that (things wrought by the Three leading to sadness) impression. In the Silmarillion, it states that wherever the Three dwelt, there also was mirth. If it had led to sadness, as you suggested, Galadriel would not have wished for the the One Ring to never have come into existance or remained forever lost, implying that she'd never go back to Valinor while Nenya lasted. Her realm was the perfect semblance of the true West.
I believe the sadness stemmed from the realization that the Three were bound to the One: should it be recovered or destroyed, the Three must fail with it. The elves were duped. I think Elrond and Galadriel were satisfied with what they could create for their realms with the Vilya and Nenya, and their respective Valinorean atmospheres. But it was the threat of Sauron and the undoing of their works that I understand was their sadness.