I suggest you leave the concrete alone. Whoever told you that 90 days is enough, I think is being optomistic. I talked to the alpine plant experts who make those concrete "troughs" for alpine garden plantings, and was told they have learned to let the concrete age for a full year before planting. I've also read that even in the rainy northwest coast (where I live, in Zone 7), plants will suffer for a year from the leaching of new concrete.
So ... don't seal the concrete, "seal" the plant! By this I mean, plant it in a large container and the toxic soil won't affect it. If you don't want the container to show, just sink the container into the dirt. This will also allow you to try a few different plant choices, and see what you like best.
One of the problems for this spot is that plants grow in a circle, unless it is somthing like boxwood or yew that likes to be sheared. This means you'll always have a problem with the plant interfering with anyone using the walkway. You might consider a vine on a trellis. There are lots of beautiful short flowering vines that will work for full sun, depending on your climate zone. And that narrow little hot zone right close to the house would be a great place to put in your kitchen herb garden, as many like a hot dry summer and lean soil. You can start with a row of pots until the concrete leaches out, then plant them next spring ... and when you are cooking, you can collect fresh herbs without trudging out to the large garden. For the shady spot you might consider a climbing hydrangea, if it is hardy in your zone.