Slugs or snails would have been our first guess, though if you're not finding any culprit when you check, it's hard to say what to try next. The approaches you've already used should have covered most chewing insect pests and, while it was dry at least, DE should help repel slugs/snails as well. (Once DE gets wet, it's not very effective.) Neem is only effective on insects it contacts directly, so avoid spraying when nothing appears to be present, since it will break down before it can work. (Dried residues aren't insecticidal.)

Have you tried using something like a terra cotta plant saucer, laid upside-down on the ground near the plant, to entice any slugs/snails to shelter under it when they are not feeding? That way, they might be easier to detect, both to remove and to identify the culprit. We don't see any tell-tale slime trails, but that might be just the angle of the photos. The spots pictured on the leaf underside do not look concerning, at least. Some beetles or other insects also do feed at night, and earwigs can be more abundant in damp conditions, but either might also take advantage of a shaded place to hide during the day, so the slug "trap" of the inverted saucer might help to identify them as well, at which point you can narrow-down a particular treatment to use for the plant.

Miri