Very interesting questions, indeed.

BTW, just another ruling by the ECJ (C-146/20):

"Article 2(b) of Regulation No 261/2004 must be interpreted as meaning that an air carrier may be classified as an ‘operating air carrier’ within the meaning of that provision in respect of a passenger if that passenger has concluded a contract with a tour operator for a particular flight operated by that air carrier without that air carrier having confirmed the hours of the flight or without that tour operator having made a booking for that passenger with that air carrier."

Moreover, the ECJ explicitly states (in number 61):

"It should also be stated that, in cases where the operating air carrier is required to pay compensation to passengers pursuant to Regulation No 261/2004 due to the conduct of the tour operator, that carrier has the possibility to seek compensation from the tour operator for any damage suffered in accordance with Article 13 of that regulation."