AFAIK, Oman Air now has the Apex suites in all of their aircraft. The window seats offer a great degree of privacy, get one of these if traveling alone. The seats may appear as 2-2-2 on the seat map, but they are individual suites with direct aisle access.
And what does "FRA-MCT on PY" mean - Oman Air does not offer Premium Economy at all, they do not have a single aircraft featuring PE seats. It's F, J and Y with them. Not that you have been downgraded to Economy....
Aeroplan's responsibility is to get you from point A to point B in the cabin class you paid for so I`d contact them and see this is rectified. Since the segment in question starts out of the EU, EC261 regulations applies and you cannot simply be downgraded without compensation.