I think that your problem is related to your very specific part. Originally it was an assembly. The assembly was exported somehow and imported as single part. Then some external merge features were created. I guess that Default Rep is available in native assembly, only.
Came across this when we saw a similar issue in a part in Creo 4 M060. The user told me the part had been imported/exported several times between neutral formats and different CAD systems.
We were able to add a new rep named "Default" and it seems that it overwrote the existing Default Rep, making it editable again.
Thought I'd share in case someone else runs into this again