Allegory in its most general form is an extended metaphor that runs throughout a work; whereas an image is localised. Neither depends on the Idea - they are affective rather than intellectual. Ubuweb positions Conceptual poetry on the Idea in opposition to Romanticism with its positioning on the Emotion.
Alan Davies poem a an av es belonging to the Language poetry school starts off as:
an anemone or we cones careen
warm venom veers on eve
sonorous moves nor arrears
concave saver screw concur
rare enamor renew ass masseuse
cure uncommon seamen excursus
It starts with sentences that gesture to the traditional structures of of poetry - sentences and alliteration and ends with just bare words it appears placed at random.
He states, in his essay, Notes on Conceptualism that
Conceptual poetry is mainly about unearthing neuroses in the minds of the people who make it.
By far and away / the most common of these is obsessive-compulsive disorder.
But mitigates this (self)-accusation by asking:
The neuroses unearthed in the process of making conceptual poetry / might (also / or alternately) be neuroses in the language.
Indeed can language induce neuroses? One could imagine several interesting but unethical experiments to test this proposition.
So, to your questions:
If an allegory does not build to an image then what is its meaning?
An allegory that is without meaning isn't an allegory; one might as well ask suppose the colour red is not red; For example, Spensor explicitly allegorises his major work The Faerie Queene around Christian themes of sin, virtue and redemption; or one could consider Robinson Crusoe who implicitly allegorises the Protestant work ethic (a la Weber) as well as the biblical Genesis; or for a more contemporary example King Kong allegorises the racial experience of America (from a white perspective) and Ellisons Invisible Man which does the same (from the black perspective).
Is it true that aesthetically and ethically the words that make a symbol [and / or allegory] are the same as its sense?
No - neither ethically or aesthetically. A symbol, such as the Rose in Yeats who drew on European symbolism to craft a new poetic idiom out of the folk and myths of Celtic Ireland - the uncreated conscience of his race. One cannot identify the symbol Rose with the word Rose; as one can verify by checking a good dictionary; The symbol Rose is its sense; it is a new sense of that word that is built by reference to his ouevre.
Frege was interested in the logical structure of language and its relation to the world of facts; whereas Conceptual poetry, building on its roots from Dada is exists:
in order to distort reality.
and not to reflect it as in a mirror without the mediating human conscience; ie the logical reflection of Wittgenstein.
The distortion of reality is more aligned with Kantian idea of the apperception of reality; and thus the phenomenological tradition.
A good reference for all the aesthetics of this is the book The Triumph of Anti-Art: Conceptual and Performance Art in the Formation of Post-Modernism by the Art Critic & Classicist McEvilley.