I've looked everywhere for a def for 28a and the best I can come up with is 'down' from 'cut down' in 8d ('down [3] a bank of sand thrown up by the sea' - it's an alt spelling of 'dune'). This gives a different entry to the one that's suggested by 'do' (I agree - 'do' is required for the def at 18a). All the same, I prefer the 'do' option - I just can't find a def for it in any of the other clues.
As it is, I don't like the 'down' option - it isn't strictly 'extra' because the clue makes sense with it or without it (then again, I think there's a word in 26a that should be 'extra' but is being used in the sense of, 'creating' as part of the surface). Also, I don't think the def is precise enough.
Hope someone comes up with a better option - it's too messy as it is (but I think a couple of Pabulum's recent efforts have been messy).