Good question. Assuming my position for now, one could argue that it’s simply stylistic (i.e., the narrator liked variety, and this would form a chiasm of plural [26], singular [27a], singular [27b], plural [27c]). Or, one could add a theological nuance: Taking advantage of the collective noun, the narrator wants to emphasize that image of God applied individually and corporately (counter-intuitively, the plural focuses on individuals; the singular focuses on corporate). This is done elsewhere. One thinks of the “seed” (another collective noun) in Gen 3:15 and 22:17-18 (though this example may help your case), or in another vein the David’s “son” in the Davidic covenant (2 Sam 7), which refers both to a succession of sons (i.e., dynasty) and to the greater Son. My own work in Deuteronomy shows the tension in the alternation of the singular “you” vs. the plural “y’all” (perhaps the only time Southern English is helpful!). The point is there could be a good explanation for the phenomenon within the traditional reading.
I’d have to think about this more on whether there’s a real distinction intended. It’s also possible different phrases are being used without too much distinction (though, I admit, it’s possible depending on authorial intent). The main differences with Day 6B (humans) that I see are: more time/text spent on it; divine deliberation (“Let us”); the lacking “according to their kinds”; the status as image of God; the added “very good” which is also delayed as a reflection on all 6 Days and not just Day Six (though “good” lacking in Day 2 since, as Waltke quips, God hates Mondays too!); and the use of the definite article “the sixth” (it also appears on “the seventh” in 2:3, but not Days 1-5; though I’m not quite sure the significance of it; I wonder what this does to our previous discussion of the article).