Thanks so much for this analysis! Saṅkhāra occurring in the singular is an important clue, as is the use of citta, presumably in the Abhidhamma sense of mind moment. The commentary seems to support your view. Here is the Ñāṇamoli’s translation of the commentary’s first paragraph relating to the Abhidhamma method:
The Master, who has unobstructed knowledge regarding all states, has thus shown in the Suttanta Division by way of plurality of consciousness the structure of conditions freed from knots and tangles, as though spreading out the great earth and as though extending space; and now, because this structure of conditions exists |200| not only in a plurality of consciousnesses but also in a single consciousness, he said avijjäpaccayä sahkhäro (“with ignorance as condition, a formation [arises J”) and so on, thus setting forth the schedule in order to teach, as to its various aspects, the structure of conditions of a single conscious moment by means of the Abhidhamma Division.
This is an important corrective to what I suggested in the other thread. However, important questions remain. First, is this really about DO in the sutta sense, or is it about conditionality in a broader sense? The Chaṭṭasaṅgāyana Tipiṭaka (the official Burmese version) has the heading Paṭiccasamuppāda-vibhaṅga for the relevant chapter, but other versions (PTS and Thai) have Paccayäkära-vibhaṅga, “Analysis of the structure of conditions”. My immediate guess is that the latter is more likely to be original, simply because it is less intuitive. If this is correct, then the Abhidhamma Analysis may be about using the structure of DO to elucidate a different area, and should not be seen as an alternative understanding of DO. This would mean that the modern tendency to see DO as a moment to moment phenomenon is still a recent development.