I feel much better about the garden.
Z. seems to know everything about horticulture. I haven't known him very long, so it's possible he's a psychopath posing as someone who knows everything about horticulture, but my intuition tells me he's for real.
He told me the small patches of mildew on the cucumber leaves are minor, nothing to worry about. The one soybean plant whose leaves are turning yellow can be doctored by putting some coffee grounds around the base of the plant.
The herbs will be fine when it gets warmer and drier. Basil always gets brown spots on some leaves, they just look worse now because the plants are so small. The beetles on the Meyer lemon are, as I hoped, probably there to eat aphids. At any rate, it doesn't look like they're eating the tree.
And the yellow spots on the watermelon leaves are supposed to be there. I planted an heirloom variety called Moon & Stars which has yellow spots on the inside and outside of the fruit as well as on the leaves. Duh.