• [B1] {naku mi tirna su'o da}I don't hear anything.
• [C1] {(da'o) naku mi tirna ko'a}I don't hear [whatever it is I don't hear].
I think the equivalence should be ⟪su'o da zo'u naku mi tirna
da⟫ = ⟪(da'o) naku mi tirna ko'a⟫ (as top-level sentences), i.e.
with ⟪su'o da⟫ taking the topmost scope. Actually it should even
scope over the illocution of the sentence (i.e. assertion,
question, command…), because, as you shown in your example
sentences with the interrogative illocutionary marker ⟪xu⟫, ⟪da⟫
is bound under the scope of ⟪xu⟫, unlike ⟪ko'a⟫. A better
equivalence would therefore be:
⟪ca'e mi do brireisku be lo ka (ce'u) jezyje'u lo du'u do tirna
ko'a⟫
= ⟪ca'e su'o da zo'u mi do brireisku be lo ka (ce'u)
jezyje'u lo du'u do tirna da⟫
As for the interpretation of ⟪zo'e⟫ as a function as you
suggested, this is reminiscent of the experimental cmavo ⟪zo'ei⟫,
by the way.
—Ilmen.