So if this is an "edge retention problem" why has nothing closer to the edge cracked loose? This looks more akin to a concrete slab cracking or tile inside a house on top of a concrete slab crack. Those cracks aren't caused by "edge retention" They are caused by foundation problems. Most people don't understand what clay soils can do to construction. Clay can swell like those toy pills you put in the "bathub". (sic) It can also shrink a lot too. All that being said, a concrete slab underneath with uber rebar and non poly sand in the joints may be the only recourse. Oh yeah and french drain it too. I would ask them to help pay for the concrete part, and if they refuse, fix it just like you built it and then you have no more liability.