I actually think that, if you have the resources, that this approach makes sense
"Do I do a fresh install of DP8, attach the brand new D2D library to it, start a fresh set of backups and leave the old DP environment around for restores?"
The one downfall is that your clients must belong to one cell or another, they can't belong to both. So, if you took this approach, you would have to restore to your local Cell Manager, then move the files to the correct client
As time goes on, and protection expires on sessions in your original cell manager, eventually, it can be retired
Regarding MOM, keep in mind that while this would aid in the goal of managing your Backup catalogs, there is additional cost involved in licensing multiple cell managers. Each one needs a Cell manger license, and a MOM license
Since so little documentation exists for MOM, I'll ask you to review the DP 5.5 Admin Guide, the last version that was published, and read Chapter 10
Sign into the SSO site (from which you posted your question), click on Manuals -> Data Protector, version 5.5, OS-type is Windows to view this