"Stoph" in :

How do I terminate the lawn as the garden drops into the patio? Flower
bed on top of the baskets?

try grit and scree loving plants. small plants that don't develop large
woody roots, of course. you might want ot wrap a layer of filter fabric
over the top of the rock, to provide separation for the layer of finer
aggregate for the grit loving plants.

interesting idea, you shouldn't have drainage problems that some retainig
walls do!