Task 7.3 suggests setting R16 and R17 as EIGRP stub to prevent the hub router R18 from sending QUERY packets in case a route goes active. I agree that this is the desired method, but there is a problem with the "servers", i.e. the VRFs on R15. If you do as the solution guide suggests, R15 loses all of his routes beyond the DMVPN spoke routers.
I originally thought I would include summary routes and advertise a default towards R15, but this breaks OSPF on R16. I'm wondering if any of you have an opinion on how to get around this problem. Perhaps we could add all the routes received via R18 into a leak-map.