I suspect that Drew wants to head toward unique identities for users (and possibly other identity types in the landscapes), but needs to fix the legacy problems first.
Generally it makes sense to fix design problems and some processes first (eg Roles...), before you implement them in IDM. Otherwise you just pass the buck on to IDM and experience the same problems / complexity there.
I cannot honestly recommend customers to use IDM to map user naming problems to identities. It is best to fix the problem and define data sources for IDM as identities (AD), and then only attributes from HR etc to enrich the business roles.
IMO it is best to first clean / simplify / harmonize the data for IDM, then implement IDM.
Cheers,
Julius