Somebody probably end-tasked a UI, or some network failure during posting.  You'll need some data repair to get past it.  

How bad of an idea to just delete the Line from OECRDH?

I don't see any mentions of the CRDUNIQ in OECRDD, OECRDDO, OECRDDD, OECRDDB, OECRDDDS, OECRDDDL, OECRDDL, OECRDDS, OECRDHO

Make a backup and try it.

copied data to test company. removed row from OECRDH.  Day end works.  Now running data integrity to see if everything is ok.  Anything else I may want to test?

Make sure there's nothing in ICHIST for that CN document.  If this was an invoice, you would have had to deal with the inventory movement from the shipment, but you should be in the clear with a CN.

The credit did not hit the ICHIST nor OESHDT tables. 

Assume that would have been part of day end, if it had worked. 

It would not hit any OESH tables, only ICHIST, ICIVAL, and the OEAUDx tables.