I think the problem here is everyone is assuming that borders were so fluid as to be unrecoverable, but this is not the case. Towns and districts were known and accepted as traditionally Mercian, or traditionally West Saxon, etc. - even if they were temporarily occupied by another kingdom. These accepted boundaries can be recovered very simply - by using ecclesiastical boundaries. Each kingdom had one or more bishoprics, and their boundaries are known (in many cases they survived unchanged until the Reformation, and any earlier or later changes are recorded).