I ended up with a pair of tiny cymbalaria muralis 'Snow Wave', which has variegated green and white leaves and flowers white.
I had 3 non-variegated cymbalaria at the beginning of the season but either I over-watered or too much sun/heat did two of them in. The survivor has quadrupled in size since late April and there's something cheerful and cute about it…enough (combined with the variegation) to coax me to try again. Fingers crossed - the $10 price tag felt really steep for such tiny sprigs.
I also got three dianthus 'Itsaul White' to add to the single dianthus I picked up at my local nursery a month or two back.
These plants looked very robust and the color and their grey spiky foliage will contrast nicely with some of the other leaf shapes and colors.
The double white balloon flower I mail-ordered at the beginning of the season was one of the plants that got off to a shaky start. It was puny and its leaves gradually browned until I didn't hold out much hope for it. At my local nursery, I noticed other balloon flower plants blossoming as mine languished. Then a couple of weeks ago it started to turn around. Now it's doing very well and the large-ish pristine white flowers are a treat. I came across three more happy balloon flower plants (these are platycodon grandiflorus 'Fairy Snow') at Avant to contribute to solving my problem of too many small-flowered plants:
Finally, as a nod to late-summer flowers, I picked up a single asteromoea mongolica.
It's very tall (3 feet now), which is something the roof garden needs near the chaise lounges and pergola, but its wispiness is useful in maintaining a sight-line across the harbor. It dances in the slightest breeze, which will either add charming movement to the roof garden or get on my nerves when glimpsed from the corner of my eye. We'll see!