HI Steve,
I can fully understand that the user cannot be synced due to the duplicate attributes (proxy address and UPN) in local AD. It is appreciated that you have tied so many methods to find the duplicate attributes, and thanks for sharing your experienced information.
As for this issue, based on our experience, we may need to use some advanced tool or specific method to find the attribute in AD. We have a group which is dedicated handling this kind issue: social.technet.microsoft.com/.../home
I suggest you post this question at that AD forum for a better response. Their engineers will be more proper and professional to troubleshoot this issue. Thanks for your understanding and effort.
Moreover, if other community members have related experience on this issue, I will appreciate it if they’ll share some information here.
Best regards,
Young Yang