I know this sounds selfish, but at 84 years of age I'm not going to be around to see if roots cause a problem with the wall, so I'm planting what suits me. As of the moment, that would be regular willows. . . edible and fast growing.Perhaps you can negotiate with the city to put in the steel barrier 6' deep ( I don't know the code there) against the retaining wall so any trees roots cannot penetrate the wall in the future.