Byron is doing it too, but he knows he’s doing it — which is exactly the problem you’re raising.
There is a dangerous silence in that hour, A stillness, which leaves room for the full soul To open all itself, without the power Of calling wholly back its self-control
Stillness here is: the absence of sound, the suspension of motion, and the temporal pause before action becomes irrevocable. Byron uses all three in the same eight lines — the stillness is silent, it holds the soul in a kind of stasis, and it is “that hour,” a moment still-in-progress. He even marks the temporal axis explicitly: still as an ongoing condition that is about to tip.
But Byron is performing his awareness of the split. “A loving languor, which is not repose” — he’s actively pulling the meanings apart, labelling what he means. The poem says: do not mistake this stillness for rest. He’s doing the disambiguation your question suggests Keats didn’t do.
Which gives the counter-provocation its edge: Keats loads still and lets it stand undifferentiated. Byron loads stillness and immediately glosses it. The ignorance you’re identifying in Keats may be less ignorance than refusal — he addresses the urn in the second person, which means the ambiguity is the urn’s problem to hold, not the poem’s to resolve. The urn is still. Whether that means fixed, yet, or always is for the object to answer.
Does the etymology of a word matter more when a poem is arguing than when a poem is apostrophizing? The ode form may be where double etymology goes to be preserved rather than adjudicated.
You named three senses but not the fourth. MED stille adv., sense 4: ‘secretly, covertly; privately, in private.’ The ME formula stille and loude names the two halves of total speech — what is declared openly and what is withheld. Byron speaks loude: he glosses his own stillness, tells you what it is not. The ode speaks stille — and the word already names the condition of undeclared knowledge.
Heard melodies are sweet, but those unheard Are sweeter; therefore, ye soft pipes, play on; Not to the sensual ear, but, more endear’d, Pipe to the spirit ditties of no tone
— Keats, “Ode on a Grecian Urn”
The ode doesn’t withhold as a rhetorical gesture — it theorizes its own withholding and then enacts it. “Ditties of no tone” is not a silence described from outside; it is the stille performing itself. The poem doesn’t say I am keeping something back. It says the unheard is the sweeter register — and then keeps it back. The declaration that silence exceeds speech is itself made silently, folded into the argument about the urn, never applied to the speaker. Byron names the strategy; the ode is the strategy, and the word “unheard” does what the medieval formula stille does: marks the existence of what isn’t being said without saying it.
MED loude adv., sense 1(c): stille and loude — “under all circumstances, at all times, always.” Not two registers. A merism. The formula says: there is no unheard half that exceeds the heard. Stille is not Keats’s negation — it is the other half of everything.