Somehow I guess so (especially if the aircraft they operate does not feature Economy Extra) - but that would be a PR disaster for SK and it doesn't cost SK any real money to upgrade you on their own flight. I am not sure how EU rules considers Economy Extra as a service class in case of overbooking.

Say, for example, that SK would short term lease an aircraft with only C and Y. I think they could get away (technically) with offer Y+ passengers a Y seat without compensation. How was this done when SK had two aircrafts in for service at the same time two summers ago due to technical mishappenings?

In the case SK transfer you to another carrier due to overbooking you get the compensation and SK usually tries to put you in C but if not possible you don't get anything extra as compensation. Just a "be happy that we can get you to your final destination" and maybe a "sorry"...

Compare a Business class ticket CPH-SEA. If you get unlucky and rerouted thorugh ORD I think you will only be offered a seat in Y on ORD-SEA since UA does not offer C-class only F. If you have Y+ you will guaranteed be offered a Y seat in domestic US.