I've worked on my share of roof top gardens as well, but never in as severe a climate as yours. I would beg to differ with Nandina on the suitability of a Japanese maple in the location as you've described it, I doubt it will ever look good. You don't say how deep your planters are, if you don't have sufficient depth and width of soil in these planters, anything that can't tolerate frozen soil at the roots will not survive the winter. That greatly limits the selection of woody or evergreen plants. In general, whatever you end up using, I'd suggest that your automatic irrigation system provide up to twice daily watering for a few minutes each cycle for best plant/root health. In general the deeper the soil depth you can provide, the greater possibilities for larger sized healthier growth. You might also consider using portable pots that can be set inside the permanent planters which would allow switching things out seasonally more easily If you're interested in red foliaged small trees, you might do better with Berberis thunbergeri cultivars or Cotinus coggygria cultivars as being far more sun/wind tolerant. Another cool looking accent shrub/small tree for you might include Poncirus trifoliata 'Flying Dragon' I'd suggest you use some hardy succulents in the mix such as Serum spectabile or S. rupestre Angelina, Sempervivums, grasses hardy to your zone. If you'd be willing to consider duoble potting your containers and bringing them inside in the winter, you could plant some truly sun/heat/wind tolerant plantings using architectural succulents such as Hesperaloes, Aloes,Agaves, Dasylirions, Echeverias, etc. These could thrive in the more temperate months outdoors and be brou__ght into a greenhouse or sun porch for the winter, and you could plunk into fall/winter seasonal accents in their place if you still needed something out there in winter, or maybe just park some cut greens and cut colorful branches such as Cornus stolonifera or willows for winter. I would stay away from any plants up there that can't stand up to the wind and hot sun and some drought stress. So no big or soft leaved plants that will wind burn.

For succulents that have been given a lot of light/harsher conditions outside and then brought inside suddenly, they can suffer from etoliation rather quickly. The cooler temperatures are very favourable, so the plants start growing, but under low light conditions they can become etoliated within just a week. There is not much to be done about it sadly and I'm not really sure if it's that preventable without very good indoor lighting (window or artificial). You can try repotting later to get it to grow straighter, but it only helps so much. My succulents are on a (fairly hot and exposed) rooftop and it took some creativity and time to get my plants adjusted to the sun - from wooden boards, to corrugated plastic, to clear diffuser panels, clay saucers, bricks, and white mesh fabric. I'm not sure if doing the same gradual increase in light/heat exposure will help your plants, but it did for me. Your shade sails look quite nice - must be comfortable under there!

Yep that’s what they are. It’s funny because on the first pic I was gonna ask what this was. You could leave them be if you want. Some will find the mix and root, while others won’t and just become dry, you can clip those. You may want to up the water a little. How much are you watering? The guy does look slightly thirsty, nothing bad tho. It could use a little plumping up. About the hairs. When I first bought mine the hairs looked more like this. pic from internet. In fact this is what mine looked like exactly. Overtime the hair has gotten much more coarser and longer. Not the best pic because the sun is gone, but you can kinda see what’s going on. The fur is not soft. I just looked at mine and they have no aerial roots. I don’t know what that means, but just curious;).

"how far from the building should I check for brachychitons? " If you ask with seed dispersal in mind that it may not matter that much, since seed dispersal agents for both, Ficus religiosa and Brachychiton populneus are birds (among other creatures), so the parent tree doesn't always have to be very close to the seedlings. There is however one more point for Ficus religiosa, and it is the leaf venation. Compare: Ficus religiosa leaves https://www.google.ca/search?q=ficus+religiosa+leaves&source=lnms&tbm=isch&sa=X&ved=0ahUKEwj9mLyq26DhAhUCvp4KHftYDzIQ_AUIDigB&biw=1526&bih=672&dpr=1.25#imgrc=_ Brachychiton populneus leaves https://www.bing.com/images/search?q=brachychiton+populneus+leaves&FORM=HDRSC2