What a nightmare!

Really sorry to hear that. Timing couldn't have been worse. Hopefully, just a matter of clarifying your risk assessments with the Local Authority events people, but boy is that going to be tough in the time available before Sunday.
Seems strange that RR would single out the Olympus. Okay, the issues with '558 are fresh in their minds, but there are "old" RR engines in taxiable aircraft all over the country - the CWJ collection and LPG for a start. No doubt they all have elements of corrosion given their age and storage conditions.
The knock-on implications of this are enormous.
Hopefully, RR and your particular LA are just being over-cautious and you can get it resolved, otherwise an awful precedent could be set, let alone the implications for MaPS 655.
Good luck and look forward to seeing you on Sunday,
Gully